[citation needed] After his service with the United States Army ended, Flanders began his acting career on Broadway before moving on to guest parts in television series.

In 1974, Flanders won a Tony Award for Best Supporting or Featured Actor in a Dramatic Presentation for A Moon for the Misbegotten by Eugene O'Neill on Broadway.

[4] In addition to his six-year role as Dr. Donald Westphall, Flanders is noted as the actor who has played President Harry Truman more times, and in more separate productions, than any other.

In the first, The Ninth Configuration (1980), he plays Col. Richard Fell, a self-effacing medic at a secret U.S. Army psychiatric facility who assists Marine psychiatrist Col. Vincent Kane (Stacy Keach).

In 1990, Flanders played Father Dyer alongside star George C. Scott in Blatty's The Exorcist III based on the novel Legion.

Flanders was in the M*A*S*H first-season episode "Yankee Doodle Doctor", playing film director Lt. Duane William Bricker, who is making a documentary about M*A*S*H units and visits the 4077th.

He also appeared in the 1979 made-for-TV-horror-miniseries Salem's Lot as Dr. Bill Norton, and played news anchor John Woodley in the 1983 made-for-TV suspense drama Special Bulletin, about a group of environmentalists who threaten to detonate a nuclear weapon in Charleston, South Carolina.

After three divorces, chronic pain from a back injury sustained in an automobile crash in 1989, and a lifelong battle with depression, Flanders died from a self-inflicted gunshot wound on February 22, 1995, in Denny, California at the age of 60.