Born in Evanston, Illinois, his mother was the pioneering African-American film actress Madame Sul-Te-Wan (née Nellie Crawford) and his father was Robert Reed Conley.

His father abandoned the family shortly after Conley's birth, leaving his actress mother to raise the three boys.

Like his mother, Onest Conley gravitated toward the acting profession and began taking bit parts in films.

Conley also appeared alongside his mother in the 1930 Richard Thorpe-directed film The Thoroughbred as the character Ham Tolliver.

His most recognizable roles include George Harris in the 1933 Cecil B. DeMille-directed crime-drama This Day and Age, Neptune in the 1935 John S. Robertson-directed romantic drama Grand Old Girl and Mose in the 1935 Sam Newfield-directed adventure film Racing Luck.