Both the Screen Actors Guild and the American Federation of Television and Radio Artists held a six-month commercials strike in 2000 that drove work overseas.
The extension comes on top of a two-year extension agreed to in 2006.
At that time, the advertising industry and the unions commissioned Booz Allen Hamilton to study new compensation models for performers to deal with the shift of ads to the Internet and other new media. The study was completed early this year.
In March, SAG and AFTRA acrimoniously ended 27 years of joint negotiations with studios over movies and prime-time TV shows in a spat over jurisdiction and the makeup of the bargaining team.
SAG and AFTRA have yet to decide formally whether to jointly bargain on the commercials contract.
The ad industry is represented by a joint committee of the Association of National Advertisers and the American Association of Advertising Agencies.