A veteran character actor known for his work on stage, screen, and television, he often portrayed comically frantic or flustered figures, becoming something of a go-to performer for such roles. On Broadway, he appeared in a number of productions, including Saratoga, The Girls in 509, and No Time for Sergeants, the last of which featured what many consider his most iconic performance. In that play, Millhollin took on the role of a beleaguered psychiatrist who finds himself outmatched by the wide-eyed innocence of a new Army recruit, played by Andy Griffith. He reprised the role in the 1958 film adaptation, bringing his signature energy to the big screen.

Throughout his film career, Millhollin became a familiar face in broad, slapstick comedies. Titles like Everything’s Ducky (1961), Zotz! (1962), Under the Yum Yum Tree (1963), Never a Dull Moment (1968), and How to Frame a Figg (1970) all showcased his knack for physical humor and comedic timing. On television, he starred in the sitcom Grindl (1963–64), playing the exasperated owner of a temporary employment agency whose life is constantly thrown into chaos by the clumsy antics of a hired maid, played by Imogene Coca. As the years went on, Millhollin continued to appear in a variety of guest roles, maintaining a steady presence in the entertainment world and reminding audiences why he was such a beloved figure in comedic acting.