Forster began writing songs while attending Harvard University in Cambridge, Massachusetts, where he earned an undergraduate degree in music. He founded the school's long-running revue The Proposition and wrote several musical shows for the Hasty Pudding Theater.

In 1988, Forster began his long collaboration with Tom Chapin as producer and chief songwriter of Chapin's Family Circle award-winning album Family Tree. He subsequently produced and wrote songs for the Chapin albums Moonboat (1989), Mother Earth (1990), Billy the Squid (1992), Zag Zig (1994) and In My Home Town (1998). Although he was not the producer, Forster also wrote several songs for Chapin's 1996 album, Around the World and Back Again. His association with the Olsen Twins began with the 1992 album Brother For Sale and continued with the albums I Am the Cute One (1993) and Give Us a Mystery (1994). Forster produced and wrote songs for Rosenschantz's albums Family Vacation (1989) and Uh-Oh (1990), and Bill Schantz's solo effort Animal Tales (1991).

Forster's lyrics were heard in the Broadway show Into the Light in 1986, and his songs were in the off-Broadway shows Upstairs at O'Neals and Pretzels, and the Goodspeed Opera Company production The Dream Team. Forster also wrote songs for five musicals produced by Theaterworks U.S.A. -- Teddy Roosevelt, First Lady, Right in Your Own Backyard, Freaky Friday and How to Eat Like a Child.

As a performer, Forster released Entering Marion, which received a honorary mention from the National Association of Independent Record Distributors in 1994, and Helium in 1997. The Official Bootleg Album, released on Forster's own Limousine lable in 1998, featured satirical songs that he had recorded but had not previously released.