He supported Dorothy Lamour in Rainbow Island (1944) and Alan Ladd and Loretta Young in And Now Tomorrow (1944), and was one of many Paramount names in Duffy's Tavern (1945). He was signed to a long term contract by Paramount, who gave him a good support role in an 'A' film, the musical Lady in the Dark (1944) with Ginger Rogers.

Sullivan had a supporting part in High Explosive (1943) for Pine-Thomas Productions, who released through Paramount, and he was the second male lead in The Woman of the Town (1943) with Claire Trevor. Sullivan had a small role in the Universal serial The Green Hornet Strikes Again! (1941). Sullivan lasted only two episodes before the show was cancelled. In 1950, Sullivan replaced Vincent Price in the role of Leslie Charteris' Simon Templar on the NBC Radio show The Saint. Sullivan appeared with Bette Davis on stage in 1960 in The World of Carl Sandburg as a substitute for her husband Gary Merrill. He was also in Mr Big (1941), Ring Around Elizabeth (1941) and Johnny 2 X 4 (1942). He received attention when he joined the cast of the long running The Man Who Came to Dinner (1939) as Bert Jefferson. He returned to Broadway with roles in All That Glitters (1938) and Eye on the Sparrow (1938) (with a young Montgomery Clift). Sullivan appeared in shorts such as Strike! You're Out (1936), Broker's Follies (1937), Dime a Dance (1937) (alongside Imogene Coca, June Allyson and Danny Kaye), Dates and Nuts (1937), and Hi-Ho Hollywood (1937). Sullivan's first appearance on Broadway was in I Want a Policeman in 1936. Career Broadway stage, film shorts and radio During the later Depression years, Sullivan was told that because of his 6 ft 3 in (1.9 m) stature and rugged good looks he could 'make money' simply standing on a Broadway stage. He fell into acting when in college playing semi-pro football. 2.1 Broadway stage, film shorts and radioīorn in New York City, Sullivan was a law student at New York University and Temple University.

Rainbow island 1944 lobby