His early career was shaped by stage performances and television roles that showcased his understated intensity. A pivotal moment came in 1962 when he appeared in "To Kill a Mockingbird," portraying the reclusive Boo Radley with remarkable subtlety. Though the role was brief, it left a lasting impression and signaled the arrival of a serious dramatic talent.

Throughout the 1970s, he became known for complex portrayals of morally conflicted and authoritative characters. In 1972, he portrayed Tom Hagen in "The Godfather," reprising the role in "The Godfather Part II." In 1979, he delivered one of his most memorable performances as Lieutenant Colonel Bill Kilgore in "Apocalypse Now," a role that became iconic in American cinema.

He continued building his reputation with powerful performances in "The Great Santini," earning critical acclaim, and later in "Tender Mercies," for which he won the Academy Award for Best Actor. In 1997, he wrote, directed, and starred in "The Apostle," further demonstrating his range as a filmmaker and performer.

In the decades that followed, he continued to take on diverse roles. He appeared in the television miniseries "Lonesome Dove," earning widespread praise, and later starred in films such as "Open Range" and "The Judge."