He is mainly for his roles as John Cassellis in Haskell Wexler's Medium Cool (1969), Lebanese terrorist Abdul Rafai in The Delta Force (1986), and Max Cherry in Quentin Tarantino's Jackie Brown (1997), for which he was nominated for the Academy Award for Best Supporting Actor.
Forster's varied filmography further includes titles such as Reflections in a Golden Eye (1967), The Black Hole (1979), Alligator (1980), Me, Myself & Irene (2000), Mulholland Drive (2001), The Descendants (2011), Olympus Has Fallen (2013), London Has Fallen (2016), and What They Had 2018).
He also had prominent roles in television series such as Banyon (1971–1973), Heroes (2007–2008), and Twin Peaks (2017). From 2012 to 2018, he had a recurring role as Bud Baxter, the father of Tim Allen's character, Mike Baxter, on the sitcom Last Man Standing, who opens a cannabis dispensary in Colorado with some help from his son.
Forster died on October 11, 2019 at the age of 78, the same day El Camino premiered. The Better Call Saul episode "" in which Forster appeared posthumously was dedicated to "our friend Robert Forster."