With his Yale degree and uppercrust aura, Dillman began on the New York stage after completing a tour with the US Marines in Korea. He made his professional acting debut in 1953 in "The Scarecrow" and spent several seasons with the Sharon (CT) Playhouse before his Broadway debut in the O'Neill play in 1956.

Within two years, the handsome dark-haired actor had been signed to a contract by 20th Century Fox. Dillman made his film debut in the soap opera "A Certain Smile" (1958), earning a Golden Globe Award as Most Promising Newcomer as a Parisian student who loses his girl to Rossano Brazzi. With "Compulsion", he seemed poised for a star career but somehow the right roles never seemed to materialize. He was well-cast as the title saint in Michael Curtiz's religious epic "Francis of Assisi" and as an espionage agent who breaks under torture (opposite future wife Suzy Parker) in "Circle of Deception" (both 1961). Subsequently, Dillman drifted into leads into lesser films and second leads in A-list productions, such as his roles as Robert Redford's college buddy and later Hollywood producer in "The Way We Were" (1973), John Wilkes Booth in "The Lincoln Conspiracy" (1977) and a reluctant hero in "Piranha" (1978). By the end of the 80s, he had all but abandoned features.

On the small screen, Dillman began acting in such anthology series as "Kraft Television Theatre". He garnered an Emmy nomination as the titular soldier in "The Voice of Charlie Pont" (1962) alongside Robert Redford. Four years later, he starred as a officer-lawyer in his first series, "Court-Martial" (ABC). In "King's Crossing" (ABC, 1982), Dillman was an alcoholic father and teacher trying to put his life straight and then spent a season (1982-83) on the CBS primetime soap "Falcon Crest". Over the years, the actor has made innumerable episodic guest appearances on show ranging from Westerns to medical dramas, generally playing either ne'er-do-wells or heroic figures. More recently, he made annual appearances (as different characters) on CBS' "Murder, She Wrote". He has also accrued over two dozen TV-movie credits, in thrillers like "Five Desperate Women" (ABC, 1971) and drama such as "Heart of Justice" (TNT, 1993). He received a Daytime Emmy for his appearance in the "ABC Afternoon Playbreak" production of "The Last Bride of Salem" (1975).

In 1995, Dillman launched a secondary career as an author. A long-time football fanatic and San Francisco 49ers fan, he wrote "Inside the New York Giants", a book which rated each player drafted by the team since 1967, with the Giants chosen as the prototype because of their commercial appeal. (Dillman has expressed his desire to update the book on a yearly basis.) Two years later, he published his memoirs, "Are You Anybody?: An Actor's Life".