Klein attended the Kansas City Art Institute and New York's Cartoonists and Illustrators School. At Marvel Comics' 1940s precursor, Timely Comics, Klein was both a penciler and an inker, initially on superhero features. He was among the pencilers of the super-speedster the Whizzer, in "All Winners Comics" #8–9 (Spring-Summer 1943). He had inked that early Marvel character, over Mike Sekowsky's pencils, as early as "All Winners" #3 (Winter 1941/42). Klein also worked on the characters Miss America (inking the premiere issue in 1944), the Young Allies, the Black Marvel, the Golden Age Black Widow, the Defender, and, under the pseudonym Nick Karlton, the Challenger. Klein found himself more utilized, however, in what was called Timely's "animator" bullpen, which created such movie tie-in and original funny animal comics as "Mighty Mouse" and "Animated Funny Comic-Tunes".