Rosen originally worked as a literary agent before he moved with his wife to the United Kingdom. He first produced the Canadian feature
A Great Big Thing (1968) and later co-produced
Ken Russell's film version of
Women in Love (1969), which won Academy Awards for
Glenda Jackson and
Billy Williams. Rosen was originally the producer of
Watership Down but took over as director after
John Hubley, the original director, left after disagreements with Rosen. He also wrote the screenplay for it. This was the first of two novels by
Richard Adams he adapted. In 1982 he also produced, directed and wrote the screenplay for another animated feature based on an Adams novel,
The Plague Dogs (1982). Rosen produced
Smooth Talk (1986), which won the Sundance Grand Prize. His last film as director was
Stacking (1987). His last project as producer was the animated
Watership Down TV series in 1999. Rosen has also worked in theater production. He was the originating producer of Michael Weller's
Moonchildren, first presented at London's Royal Court Theater before transferring to the US. He was the originating producer of Maxine Hong Kingston's "The Woman Warrior", presented in association with The Berkeley Rep, Boston's Huntington Theater, and the
Doolittle Theater in Los Angeles. ==
Watership Down ownership controversy==