A veteran stage actor, Mulhall's first serial was 1918, Universal's "The Brass Bullet". His overt enthusiasm made him a natural for serial stardom. After a flourishing career as a star in the silent era His first big serial in the sound era was Mascot's "The Three Musketeers" (1933) where he helped John Wayne clear his name of murder while tracking down the evil El Shaitan. That same year he also made "Mystery Squadron" for Mascot. Incredibly he was unmasked as the mystery villain, the Black Ace.
The following year Mascot starred him as the racecar driving "Burn'em Up Barnes" (1934) who kept a gang of hoods from taking over Lola Lane's cab company. His biggest serial was Stage and Screen's "The Clutching Hand" (1936). Mulhall assayed the role of scientific detective Craig Kennedy, who battled his old archenemy while searching for a missing formula to make synthetic gold.
After that Mulhall moved into smaller, character roles, popping up in countless serials like "Tim Tyler's Luck" (1937) and "Radio Patrol" (1937) for Universal; "Mysterious Doctor Satan" (1940), "Adventures of Captain Marvel" (1941), and "Dick Tracy vs. Crime Inc." (1941) for Republic; as well as "Blackhawk" (1952) for Columbia.
His feature film work includes "Outlaws of Sonora" (1938), "Mr. Wise Guy" (1942), "Ghosts on the Loose" (1943), and "Neath Canadian Skies" (1946) to name a few.