A prolific character actor with steely-eyes and distinctively stern features, Marlo could be counted upon to portray tough cookies and villains with consummate ease. Among a bevy of supporting roles and bit parts in 60s and 70s films and TV were occasional leads in second features, such as in Irvin Kershner's thriller The Young Captives (1959) in which he played a homicidal maniac holding an eloping couple hostage. Among numerous appearances on the small screen he could be seen as Native Americans in Death Valley Days (1952), insidious Special Investigations Department officials in Land of the Giants (1968) and as a dapper, straw boater-wearing henchman in the A Piece of the Action (1968). About his career Marlo said in a 2014 interview "I wasn't a big star, but I worked. I made a living at it. It was up and down." An outspoken champion of equal rights and social justice, Marlo's celluloid reputation as a heel certainly did not extend into his personal life.

Steven Marlo was born Morris Miller in San Francisco. After serving in the Army Air Corps he attended acting school in L.A. on the G.I. Bill but dropped out and moved to New York, temporarily making ends meet as a cab driver and dish washer while auditioning for acting jobs. In 1951, he got his first break on Broadway in the comedy play "The Royal Family". Two years later he was in the cast of "Picnic" alongside a young stage newcomer named Paul Newman. As regular theatrical work became more and more difficult to come by, Marlo relocated to Hollywood, successfully auditioned for the role of a bodybuilder in the low budget crime drama Stakeout on Dope Street (1958) and subsequently adopted his new stage moniker. Around this time he completed drama studies at the Actor's Studio under Lee Strasberg. Until his retirement in 1990 he amassed some 79 acting credits on screen, having played anything from gangsters to police officers, from medics to a deformed hunchback in Mumyalar Müzesinde Dehşet (1973).