Boxleitner was born in Elgin, Illinois, the son of a certified public accountant. He attended Prospect High School in Mount Prospect, Illinois, and the Goodman Theater School of Drama of the Art Institute of Chicago.

Television

Boxleitner is best known for his leading roles in the television series How the West Was Won, Bring 'Em Back Alive, Scarecrow and Mrs. King (with Kate Jackson), and Babylon 5 (as John Sheridan in seasons 2–5, 1994–1998). He also starred in The Gambler TV movie series (as Billy Montana, alongside Kenny Rogers: 1980, 1983 and 1987). In 2005 he co-starred as Captain Martin Duvall in Young Blades. He has also starred in several films within the Babylon 5 universe, including In the Beginning (TV, 1998), Thirdspace (TV, 1998), A Call to Arms (TV, 1999) and the direct-to-DVD The Lost Tales (2007).

He has made appearances in many other TV shows, such as The Mary Tyler Moore Show, Gunsmoke, Judith Krantz's Till We Meet Again, Tales from the Crypt, Heroes, Touched by an Angel, The Outer Limits and She Spies, and in 1982 he played Chase Marshall in the TV movie, Bare Essence, with Genie Francis. He also was a member of the cast of Heroes for seasons three and four, playing New York Governor Robert Malden in three episodes. He also appears on the television series Chuck as the father of Devon Woodcomb.

He has also been in the made-for-television films The Secret, Falling in Love with the Girl Next Door, Pandemic, The Bone Eater, Sharpshooter and Aces 'n Eights.

Boxleitner is set to guest-star on NCIS in the fall of 2010. He will play Vice Admiral C. Clifford Chase, a high-ranking Navy official.

Film

He has also appeared in several films, including Tron (in which he played the title role) and The Baltimore Bullet (1980) with James Coburn. He will reprise his role in the Tron sequel Tron Legacy, as he did for a sequel video game named Tron 2.0, and Disney/Square Enix crossover video game Kingdom Hearts II. He also starred as Confederate General James Longstreet in the 2003 film Gods and Generals. He provides the voice of Colin Barrow in the animated sci-fi horror movie Downfall, based on the video game Dead Space. Other films he has been in include Kuffs, The Babe, Brilliant, Snakehead Terror, Legion of the Dead, King of the Lost World, Shadows in Paradise and Fall of Man.

Modeling

From 1986-1989, Boxleitner appeared in advertisements for Estee Lauder's "Lauder For Men."

Novels

Boxleitner has written two science fiction novels with a Western setting: Frontier Earth (1999)

and ''Searcher'' (2001).

Audio books

Boxleitner played a major role in the audio dramatization of The Great Secret, part of the Golden Age of Fiction series by L. Ron Hubbard.

Boxleitner's first marriage (1977—1987) was to American actress Kathryn Holcomb. They had two sons together, Sam and Lee. Holcomb went on to marry English actor Ian Ogilvy, best known to American audiences as "Lawrence" on Upstairs, Downstairs. In a twist, Ogilvy also guest-starred on Boxleitner's series Babylon 5.

Melissa Gilbert also guest-starred as Boxleitner's on-screen wife, Anna Sheridan, on Babylon 5.

In 2003, Boxleitner was appointed to the Board of Governors of the National Space Society, a nonprofit, educational space advocacy organization founded by Dr. Wernher von Braun.