Always? No. The contract may explicitly provide that terms will not be construed against either party (for example in negotiated agreements such as mergers, VC, so on) or the facts may indicate a negotiated agreement where both parties were represented by counsel. The rule generally applies where the drafter is the dominant party or otherwise represented by counsel and the other is less sophisticated and not represented by counsel.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
I agree with Mr. Murillo. In addition, you should keep in mind that the court will analyze the ambiguous provision in light of the rest of the contract and the conduct of the parties. A provision may be subject to multiple interpretations standing alone, but maybe only one of those interpretations makes sense in light of the rest of the contract. In that case, the court isn't going to throw out the ambiguous provision or construe it against the drafter.
Also, if the provision is ambiguous the court can admit external evidence to clarify ambiguity. This is an exception to the "parole evidence rule."