There's been no movie made with a Montego as the 'star', but there have been a few movies with Montego's in the background. Below are some that I have found. You can see others at the Internet Movie Car Database, located at this link: , do a search on Mercury Montego.

Youtube has a classic 69 Montego/Cyclone commercial, view at the link below:
<<< Dog Day Afternoon, staring Al Pachino. Warner Studio's 1975. A bank robbery gone wrong leads to a media circus. Check out the Montego (blue 69 with a black top) in the front of the bank throughout the movie.
The Love Bug, Disney, 1968. A classic movie with a VW that has a bit too much drive, if you will. In the scene where Dean Jones and "Herbie" are being challenged by some hippies in a hot rod at a stoplight, in the background is a brand new, gold Montego MX Brougham. Thanks Jay for this one!    >>>
<<< Airport, staring Dean Martin. 1970. Here's the movie that inspired the Airplane spoofs and a series of other spinoffs. This movie also has a red 68 or 69 Montego that's shown when Dean Martin is dropped off at the airport. Thanks Dave for this one!