Barnett graduated from Duquesne University Prep School and the Carnegie Institute of Technology (now Carnegie Mellon University). An avid amateur pilot, he flew mail planes for a couple of years. The diminutive, chrome-domed Barnett appeared on Broadway in the 1926 edition of Earl Carroll’s Vanities.

He began appearing in films in 1930, playing hundreds of comedy bits and supporting parts until retiring in 1975. Among his more sizable screen roles was the moronic, illiterate gangster “secretary” in Scarface. From 1930 Barnett appeared, usually as comedy relief, in films and on television in a career spanning 45 years. Among his best-regarded early roles, apart from Scarface, were The Big Cage (1933), Thirty Day Princess (1934) and, in a perfectly suited Runyonesque part, Princess O’Hara (1935). In later years, Barnett played straight character parts, often as careworn little men, undertakers, janitors, bartenders and drunks in pictures ranging from films noir (The Killers, 1946) to westerns (Springfield Rifle, 1952). He was a welcome presence in “B” comedies and mysteries: as Runyonesque gangsters in Petticoat Larceny (1943), Little Miss Broadway (1947), and Gas House Kids Go West (1947), and notably as Tom Conway’s enthusiastic sidekick in The Falcon’s Alibi (1946). After World War II, with the Hollywood studios making fewer films, Barnett became a familiar face on television.

A 1932 newspaper report noted that “Barnett for years [was] known in Hollywood as the ‘professional ribber’ — appearing at banquets and parties as a paid ‘insulter.'” He would insult the guests in a thick German accent, spill the soup, and drop the trays—all to the great delight of hosts who enjoyed watching their friends squirm and mutter “Who hired that jerk?” Wrote author Ephraim Katz, “Among the celebrated ‘victims’ of his practical jokes were President Franklin D. Roosevelt, Winston Churchill, Bernard Shaw, Henry Ford, and Charles Lindbergh.”

During the transition from silent films to sound, an employee at MGM hired Barnett to prank Louis B. Mayer. He impersonated a sound expert and went to a sound-stage under construction with Mayer, criticizing the construction and using double-talk to confuse him. He ended his evaluation by proclaiming the whole soundstage needed to be torn down, and Mayer was about to order it done before his co-workers revealed the prank. David Niven, in his 1975 memoir, recalled Barnett posing as an important German director at a testimonial banquet for Samuel Goldwyn. Barnett gave the guest of honor an uncomfortable time, announcing that Goldwyn hired actress Anna Sten only because he “wanted to get into her bloomers.”