Skip to main content

Mark Diehl Sullivan

Litigation Attorney at Wilson, WY

4.9   30 reviews
  • State Years
    Wyoming - WY 18 years
    New York - NY 25 years
  • Licensed for 18 years
    State WY
    Acquired 2005
    Active No misconduct found
  • Licensed for 25 years
    State NY
    Acquired 1998
    Suspended No misconduct found

We have not found any instances of professional misconduct for this lawyer.


Mark D. Sullivan, P.C.
5237 HHR Ranch Road
Wilson, WY, 83014-9210


4.9 / 5.0
  30 customer reviews
5 Star
4 Star
3 Star
2 Star
1 Star
Posted by Dustin | January 9, 2024
This review is from a person who hired this attorney.
| Hired Attorney
Exceptional lawyer in ever aspect. Mark is honest and upfront eith everyone. Treats everyone in the case with respect even tje opposition. Mark is driven and always willing to take a call. If you need somome who is very tactful and will earn your business Mark is the attorney you want.
Posted by Caroline | May 18, 2022
This review is from a person who hired this attorney.
| Hired Attorney
Last year, we found ourselves in a very unfortunate dispute over language in a lease we signed with a property manager and tenants. The conflict brought me to my was one of the hardest things I have ever experienced (and I have known real pain). And we could not find an attorney to help us. When we called Mark Sullivan and explained the situation he agreed to help us. Over the next few months he provided exceptional counsel, and we ultimately prevailed. He was always available to us, and knew exactly when to push the other side, and when to give them time. Mark's professionalism, knowledge, wisdom and expertise helped us tremendously, and I am forever grateful.
Read less
Posted by anonymous | January 20, 2021
This review is from a person who hired this attorney.
| Hired Attorney
Property Damage Case
We hired Mark to help us manage a property damage case. We were a large group of defendants that were out of state and had never dealt with a situation like this; we hired Mark given his track record and expertise. Mark was nothing shy of phenomenal throughout the entire process. He did an incredible job coaching us along the way and giving us sound advice to navigate the legal proceedings in Wyoming. Mark is very intelligent and professional and his fees were very reasonable. Ultimately, Mark helped us get this case settled which was optimal outcome for all parties. I would absolutely recommend Mark to potential clients because of his ability to ease a very stressful process.
Read less
Posted by Kenneth | January 20, 2021
This review is from a person who hired this attorney.
| Hired Attorney
Construction Litigation
After a dispute with our builder we hired Mark to handle our case. We were very satisfied with the outcome and Glad that we chose Mark to handle this for us. He was very knowledgeable in the facts and the law and handled this very professionally and presented a well thought out and strong case. His fees were very reasonable and it was a lot less stress knowing you had a qualified person leading in what can be a stressful event.
Read less
Posted by anonymous | January 8, 2020
This review is from a person who hired this attorney.
| Hired Attorney
HOA legal representative
I am the president of our homeowners Board of Directors. Mark represented us in litigation against one of our homeowners who was in violation of our covenants. He developed evidence and litigated the case in court successfully, allowing us to remedy the issue and recovered all our costs. He has always been personable, diligent, and willing and able to provide solid legal advice to our board of directors and our homeowners.
Read less
Posted by James Hadden | May 1, 2017
This review is from a person who hired this attorney.
| Hired Attorney
Very pleased.
Mark was willing to take my case when several other Wyoming lawyers showed no interest at all. He was genuinely concerned with my well being and worked very hard to build a strong case on my behalf. Personal injury cases are very difficult to win in the state of Wyoming, but he was relentless in his pursuit of a fair settlement. I believe Mark is one of a rare breed of modern lawyers who is completely honest and approaches a case with an open mind and strong conviction. I highly recommend him.
Read less
Posted by Skye | December 12, 2016
This review is from a person who hired this attorney.
| Hired Attorney
Great lawyer, very helpful in car crash settlement
I was very glad that I hired Mark as my lawyer after a nasty car crash last winter. He made a complicated process very straightforward, explained what was happening, and was a strong advocate for me all along. Definitely recommend and would get Mark's help again in future.
Posted by Trip | December 6, 2016
This review is from a person who hired this attorney.
| Hired Attorney
Wyoming Workers Compensation Excellence
Mark Sullivan helped me navigate the Worker's Compensation process from start to finish. He helped explain the process making it easier to understand. He is responsive and efficient. Mark remained my advocate compassionately explaining to me different choices through a lengthy process. He was patient and diligent despite my difficulties with memory, spinal surgeries and extenuating circumstances. Ultimately, we prevailed. I cannot speak more highly of his character, legal proficiency and tenacity.
Read less
Posted by david | October 28, 2016
This review is from a person who hired this attorney.
| Hired Attorney
Mark Sullivan by David
Mark did a great for my wife and myself. If we ever need a lawyer for any other work he would the first person I would call and hire. He did a very good job with keeping us informed as to what was going on at all times. When it came time for the trail he was one of the most organized lawyers I ever saw.
Posted by Elizabeth | October 12, 2016
This review is from a person who hired this attorney.
| Hired Attorney
I hired Mark Sullivan to deal with a situation we had with a builder. He was extremely honest and very easy to work with. He settled the matter for us quickly & fairly. We are very happy with the outcome. I would not hesitate to recommend Mark to anyone.
Posted by Shaun Maloney | June 24, 2016
This review is from a person who hired this attorney.
| Hired Attorney
Actually he deserves 10 stars!
Mark recently was my lawyer in a Workman's Comp hearing. I have been working with Mark for several months in preparation. At the hearing last Tuesday, June 21, 2016, Mark represented me honestly, efficiently and honorably. I would have no problem recommending Mark as I respect him as a human being and lawyer. He was always promptly at our disposal over the past few months and prepared me for the hearing. Mark is truly a gentleman and a darned good lawyer!
Read less
Posted by anonymous | December 7, 2015
This review is from a person who hired this attorney.
| Hired Attorney
Mr. Sullivan is my company's attorney and has litigated cases as they arise very successfully. He is always available to review issues as they arise. His work has helped us win suits from customers and saved us from unjustified refunds.
Posted by Lisa | October 28, 2015
This review is from a person who hired this attorney.
| Hired Attorney
Mark Sullivan has my highest recommendation, without reservation.
Mark is an excellent attorney: professional, intelligent, thorough and perceptive. He managed our case with a clear game plan, his writing was, without exception, prompt and impeccable, with close attention to detail. Mark was always knowledgable and prepared, clearly outlining issues and options at every turn. His calm demeanor inspired great confidence throughout the legal process. Mark is so easy to work with that I plan to contact him for any legal matters that arise in the future. A smart, straight shooter and a good man.... I can recommend Mark without any hesitation whatsoever.
Read less
Posted by John | October 27, 2015
This review is from a person who hired this attorney.
| Hired Attorney
Grateful Attorney from Texas
Mark did an exceptional job on a dispute I had with a construction contractor . I have been a licensed lawyer for 30+ years and Mark served me well with good advice and action. I’d highly recommend this gentlemen to anyone with a litigation problem in Wyoming.
Posted by Kimball | October 18, 2015
This review is from a person who hired this attorney.
| Hired Attorney
Mark Sullivan is a first class lawyer. I highly recommend him! Mark successfully represented me in a Workers Compensation action, displaying dogged determination, keen anaylitic skills, a thoroughly researched knowledge of the law and confident courtroom demeaner.
Posted by Stephanie | October 1, 2015
This review is from a person who hired this attorney.
| Hired Attorney
Happy with the result of my workers comp case, he did a great job as my lawyer. He's very understanding and patient. Definitely hire him to be your lawyer and you're going to be in good hands.
Posted by Lonnie | November 13, 2014
Mark did a very good job of educating me in the process. He was instrumental in getting what I needed from Wyoming Worker's Compensation. He was very knowledgeable and provided fantastic arguments. He contacted me when said he would and conducted himself as nothing but professional. I think his patience with me was more than exceptional. I questioned everything and he knew the answer or would find it for me. I would and have recommended his services to others.
Read less
Posted by Carole | November 8, 2014
Mark Sullivan tackles the most frustrating legal issues with skill and integrity.
Our organization hired Mark Sullivan four years ago. We needed legal advice on some longstanding and ongoing issues. We were impressed with how quickly he grasped the facts and provided options. Mark did not try to rush us to litigate but rather encouraged a non court solution. Once it was clear that was not to happen, he advised us on other paths. Mark was clear about the issues, estimated the costs, length of time the effort might take and guided us with dignity and expertise through the processes. We prevailed in each case. Having Mark as our attorney gives us great peace of mind.
Read less
Posted by Bob | November 6, 2014
Our Go-To Lawyer
Mark has served as my company’s legal counsel for the past five years. During that time he’s helped us successfully navigate a wide range of vexing legal issues including implementing our stock option plan. I find Mark exceptionally easy to work with- he’s sharp, responsive and, when necessary, hard-nosed. Perhaps even more importantly, he knows how to rein-in my sometime irrational urge for legal confrontation when it’s not in the company’s best interest. I recommend Mark without reservation.
Read less
Posted by Thomas | November 5, 2014
Clever, Cunning, Efficient, and Excellent Communication
I can't be specific, but I hired Mark Sullivan at least three times in 5 years to help in an ongoing intellectual property dispute, which thanks to Mark is no longer "ongoing." Its over! With carefully calculated negotiations and round table toughness, Mark successfully took on a team of opposing lawyers and won for me a favorable settlement. Later he showed thorough knowledge of the law and keen resourcefulness, not only quelling opposition attacks, but also winning for me a greater reward than either of us expected. If you are looking for an attorney who knows the law, knows how to use it, and can work effectively and efficiently on your behalf, I would highly recommend Mark Sullivan.
Read less
View less reviews


  • Hourly Rates $ 150-250 per hour
  • Contingency 25%-40%
  • Retainer Sometimes
Payment methods
Cash, Check


AVVO RATING 7.0 (Very Good)

Work Experience

  • Partner
  • Mark D. Sullivan P.C.
  • 2006 - Present
  • Associate
  • Carter Ledyard & Milburn, LLP
  • 1999 - 2006
  • Senior Watershed Attorney
  • Riverkeeper, Inc.
  • 1997 - 1999


  • Pace University School of Law
  • LL.M - Master of Laws
  • 1999
  • Pace University School of Law
  • J.D.
  • 1997
  • Kenyon College
  • BA - Bachelor of Arts
  • 1993

Sample of Legal Cases

  • Shaw Construction v. Rocky Mountain Hardware, 275 P.3d 1238 (2012)
  • Victory
  • Meyer v. Hatto, 198 P.3d 552 (2008)
  • Victory
  • RISD v. RIOC, 291 A.D. 2d 40; 735 N.Y.S.2d 83 (A.D. 1st Dept 2001)
  • Victory


  • Wyoming Trial Lawyers Association
  • 2010 - Present
  • Teton County Bar Association
  • 2006 - Present


  • New York State Bar Association Journal (cover story)
  • The EPA's New Clean Air Rules
  • 2006

Attorney endorsements

Received (3)
Given (6)
Endorse Mark
Litigation Attorney | Apr 29
Relationship: Worked together on matter
"Mark is an excellent lawyer and a very amiable person. Our firm worked with him as co-counsel on two cases in Wyoming. Mark has a solid knowledge of the law, is a skilled litigator, is deeply concerned for the well-being of his clients, and is a consummate professional. I would recommend him to others looking for a lawyer to represent them in Wyoming."
Litigation Attorney | Feb 15
Relationship: Fellow lawyer in community
"Mark gets terrific results for his clients and is a really good person to boot."
General practice Attorney | Nov 04
Relationship: Friend
"I have known Mark for over 16 years and I can say unequivocally that he is one of the most naturally gifted lawyers I have ever known. His ability to craft legal arguments and understand complex legal issues is remarkable and make him a formidable adversary in and out of court. He is also a consummate professional and widely respected. Although these qualities alone would make him a good lawyer, Mark also is a committed and passionate advocate for his clients and a genuinely decent guy to boot. Accordingly, I recommend and endorse Mark without reservation."
Environmental and natural resources