He had the male lead in The Student's Romance (1935) with Grete Natzler and Honours Easy (1935) with Greta Nissen and was in Abdul the Damned (1935), Mister Hobo (1935) with George Arliss, Wrath of Jealousy (1936), and Two's Company (1936).

He was back at RKO for two more films with John Farrow: Married and in Love (1940) and a remake of A Bill of Divorcement (1940) with Maureen O'Hara and Adolphe Menjou.

He went to Republic for Women in War (1941) then was top billed in Universal's The Strange Case of Doctor Rx (1942), and Mystery of Marie Roget (1942) with Maria Montez.

He supported Irene Dunne in Gregory La Cava's Lady in a Jam (1942) with Ralph Bellamy and Eugene Pallette, Constance Bennett in Sin Town (1942), Abbott and Costello in Who Done It?

(1942) and Hit the Ice (1943), Ilona Massey in Frankenstein Meets the Wolf Man (1943) with Lon Chaney Jr. and Bela Lugosi, Rosemary Lane in All by Myself (1943), The Andrews Sisters in Always a Bridesmaid (1943), Olsen and Johnson in Crazy House (1943), Donald O'Connor and Peggy Ryan in Chip Off the Old Block (1944) and This Is the Life (1944), and Gloria Jean in Pardon My Rhythm (1944).

[10] Knowles went to Paramount where he supported Paulette Goddard and Ray Milland in Kitty (1945), Dorothy Lamour in Masquerade in Mexico (1945), Barbara Stanwyck in The Bride Wore Boots (1946), and Alan Ladd in O.S.S.

[13] Knowles went to RKO for The Big Steal (1949) with Robert Mitchum, Jane Greer and William Bendix, and 20th Century-Fox for Three Came Home (1950), second billed, playing Claudette Colbert's husband.

He still appeared in features such as Quebec (1951), Mutiny (1952), Tarzan's Savage Fury (1952) (as the villain), Jamaica Run (1953), Flame of Calcutta (1953) for Sam Katzman (second billed to Denise Darcel), World for Ransom (1954), Khyber Patrol (1954) and No Man's Woman (1955).

[15] From the late 1950s Knowles became an almost exclusively television actor appearing in Star Stage, The 20th Century-Fox Hour, Tales of the 77th Bengal Lancers , Ethel Barrymore Theatre, Lux Video Theatre, Matinee Theatre, Schlitz Playhouse, The Millionaire, Lux Playhouse, Walt Disney's Wonderful World of Color, Wagon Train, General Electric Theater, 77 Sunset Strip, Tightrope, Maverick in episodes "The Wrecker" with James Garner and Jack Kelly and "Guatemala City" with James Garner, The Barbara Stanwyck Show, Klondike, The Jim Backus Show, Death Valley Days, The Islanders, Checkmate, Peter Gunn, Whispering Smith, Hawaiian Eye, Have Gun – Will Travel (as Phileas Fogg in the episode "Foggbound", and as August in “Savages”) with Richard Boone, Gunsmoke, The Rogues, Mickey with Mickey Rooney, and Jericho.

He had a small role in the feature Band of Angels (1957) with Clark Gable and Sidney Portier, From the Earth to the Moon (1958) and Auntie Mame (1958) with Rosalind Russell.

: Murder One (1969), Getting Together (1971), The Man (1972) with James Earl Jones, Terror in the Wax Museum (1973) with Ray Milland and Elsa Lanchester, and Arnold (1973) with Stella Stevens.