Lange was born in Oakland, California, in 1948, the son of Geraldine L., a television show host, and Ted Lange. Lange attended the Royal Academy of Dramatic Art. Lange was a cast member of the musical Hair. His first screen appearance was in the documentary film Wattstax in 1973. After appearing in the film Black Belt Jones in 1974, he portrayed Junior on the series That's My Mama before landing the role of the ship's bartender, Isaac, on The Love Boat in 1977, opposite Gavin MacLeod. After he left the show in 1987, Lange appeared in various films and guest roles on 227, The Cleveland Show, Glitch!, Evening Shade, Scrubs, Drake & Josh, The King of Queens, Boy Meets World, Psych and Are We There Yet.
In addition to his film and television work, Lange has also done extensive theater work. He made his Broadway debut in 1968 in the musical Hair. He also performed in a one-man show, Behind the Mask: An Evening with Paul Laurence Dunbar.
During the run of The Love Boat, Lange also served as director and screenwriter on various episodes of the series. In 1977, he wrote the screenplay for the 1977 drama Passing Through, starring Cora Lee Day and Marla Gibbs. In 1999, Lange directed two episodes of Love Boat: The Next Wave, the UPN series based on The Love Boat. He also directed episodes of Moesha, Dharma & Greg, and Eve. In 2008, he directed the drama For Love of Amy.
Lange has also done extensive theater work as playwright and stage director. He has penned 17 plays, including George Washington's Boy, a historical drama about the relationship between the first president and his favorite slave, along with the comedy Lemon Meringue Facade.
Lange remains close to Gavin MacLeod, his acting mentor, who is a Palm Springs resident, and sees his plays. He said in a 2014 interview with CBS New York.com of his long-running friendship with him, "Gavin lives in Palm Springs, I'm in LA. So, when I do my plays, he comes down and sees my plays or I'll go see what he's doing!" He also said in a 2017 interview with The Wiseguyz Show, if his mentor enjoyed all the acting/dancing on The Love Boat series was: "Oh yeah, sure, Gavin was wonderful. Gavin lives down here in Palm Springs and we're still tight, all of us, Gavin and Bernie and Jill; we still see each other. Fred lives in a different state, we're still close, we're still good friends."
Before the American edition of FHM folded in 2006, Lange wrote a sex and advice column, titled "Ask Isaac", with adult film actress Jenna Jameson.
In 2006, Lange appeared in the fourth season of the VH1 reality show Celebrity Fit Club. He lost 28 pounds during the show's run.
Lange married Sheryl Thompson in 1978, and they divorced in 1989. The couple has two children, Ted IV and Turner Wallace Lange. Lange remarried in 2001 to Mary Ley. His mother is Geraldine Lange, who was a personal secretary to a San Francisco mayor and was public affairs director of KBHK-TV in San Francisco in the early 1970s. She also hosted television programs on KBHK-TV.
For his work theater directing, Lange received the NAACP's Renaissance Man Theatre Award, the Heroes and Legends HAL Lifetime Achievement Award, and the Dramalogue Award. Lange has also been the recipient of the James Cagney Directing Fellow Scholarship Award from the American Film Institute along with the Paul Robeson Award from Oakland's Ensemble Theatre.