Sinnott's first collaboration with Jack Kirby, one of comics' most historically groundbreaking and influential creators and the penciler with whom he is most often identified, came with the war-comics story "Doom Under the Deep" in Atlas' "Battle" #69 (April 1960). After a supernatural Kirby story in "Journey into Mystery", #58 (May 1960), he inked Kirby's twice-reprinted giant-monster story "I Was Trapped By Titano the Monster That Time Forgot" in "Tales to Astonish" #10 (July 1960), although not the cover featuring that lead story. Sinnott in 1992 believed his first Kirby collaboration was a Western story titled "Outlaw Man from Fargo", but nothing approximating that appears in standard databases.