The New York-born star started his career on Broadway in Mrs Dally Has A Lover before being cast in Reflections In A Golden Eye opposite Elizabeth Taylor and Marlon Brando in 1967.
Over the course of his career, Forster starred in more than 100 films including Jackie Brown, Charlie's Angels: Full Throttle, The Descendants and Me, Myself and Irene.
His role in Jackie Brown even earned him an Oscar nomination in 1997.
Forster also appeared in dozens of TV shows, including Breaking Bad, Twin Peaks, Heroes and Last Man Standing.
Breaking Bad star Bryan Cranston was among the first to pay tribute to Forster in an emotional post on Twitter.
There's also been a strong outpouring of grief among fans.
He is survived by his four children Bobby, Elizabeth, Kate and Maeghan; his grandchildren Tess, Liam Jack and Olivia; and longtime partner Denise Grayson.