Because he was on staff, Klein frequently did not sign his artwork – a typical though not ironclad industry habit at the time – making it difficult to assess his Golden Age output.

[5] In 1955, Klein began his long association with penciler Curt Swan on a variety of titles in DC Comics' "Superman family", edited by Mort Weisinger.

Starting with uncredited but generally recognized inks over Swan in Superboy #38 (Jan. 1955) – on a backup story featuring the Boy of Steel vs. "Public Chimp Number One!"

Later in the 1960s, Klein became the chief inker on Adventure's lead feature, the Legion of Super-Heroes, by writer Jim Shooter and penciler Swan,[5] helping set the visual foundation for what would become one of DC's most popular series.

In 1968, with new art director and soon-to-be editorial director Carmine Infantino given the mandate to revitalize DC in the wake of rival Marvel's pop-cultural and industry ascendancy,[citation needed] Klein was eased out along with such other Superman-family artists as Wayne Boring, Jim Mooney, and George Papp, and writers Otto Binder, Edmond Hamilton, and Jerry Siegel (Superman's co-creator with Joe Shuster).

Before the mid-2000s and the maturity of comics scholarship, inking credit for the landmark issues The Fantastic Four #1–2 (Nov. 1961 – Jan. 1962) was generally attributed to Dick Ayers, a frequent Kirby inker before and after.

The standard Grand Comics Database, for example, unequivocally credits Klein as the primary inker (with possible alterations by Sol Brodsky) based on the analysis of Michael Vassallo and Nick Caputo.

He embellished John Buscema on a run of The Avengers; Gene Colan on issues #46-49 off that penciler's signature series, Daredevil; and, in his last assignment, Jack Kirby on Thor #168–169 (Sept.-Oct.

[5] Among the Silver Age issues he inked were the Avengers stories that introduced the Vision, Yellowjacket, and the Clint Barton Goliath, and another with the marriage of Henry Pym and the Wasp, Janet Van Dyne; "Brother, Take My Hand" in Daredevil #47 (Dec. 1968), cited by Stan Lee as one of his favorites among the comic-book stories he wrote;[citation needed] and the cover and interior of one of Barry Windsor-Smith's first U.S. comic books, Daredevil #51 (April 1969).