Singleton began her show business career when she was a child, singing at a silent movie theater, and toured in vaudeville as part of an act called “The Kiddie Kabaret”. She sang and danced with Milton Berle, whom she had known since childhood, and actor Gene Raymond, and appeared on Broadway in Jack Benny’s The Great Temptations. She also toured in nightclubs and roadshows of plays and musicals. Singleton appeared as a nightclub singer in After the Thin Man, and was credited at this time as Dorothy McNulty. She was cast opposite Arthur Lake (as Dagwood) in the feature film Blondie in 1938, based on the comic strip by Chic Young. They repeated their roles on a radio comedy beginning in 1939 and in guest appearances on other radio shows. As Dagwood and Blondie Bumstead they proved so popular that a succession of 27 sequels were made from 1938 until 1950 with the radio show ending the same year. Singleton’s husband Robert Sparks produced 12 of these sequels. Singleton dyed her brunette hair blonde for the rest of her life.

Singleton won “Top Billing” in Go West, Young Lady over her male co-star, Glenn Ford — putting her in the elite company of only two other female stars (Dorothy Page and Jane Frazee) who held the headliner roles as top-billed singing cowgirls. She starred as Adelaide in Guys and Dolls at the summer Starlight Theatre in Kansas City, Missouri in 1954. She was active in union affairs and was the first woman president of an AFL–CIO union. In 1967 she led a month-long strike by the Radio City Rockettes for better working conditions, which they won. She became familiar to television audiences as the voice of Jane Jetson in the animated series The Jetsons, which originally aired from 1962 until 1963, reprising the role for a syndicated revival from 1985 through 1988 and for assorted specials, records, and Jetsons: The Movie (1990). Singleton died in Sherman Oaks, California, following a stroke and was interred in San Fernando Mission Cemetery in Los Angeles, California. She was married to Dr. Laurence Scogga Singleton, a dentist, from 1937 until their divorce in 1939. She was married to Robert Sparks from 1941 until his death on July 22, 1963. Singleton had a daughter with each of her husbands.