In August of 1990, Tomlin was recalled to replace Rick Reed in Pittsburgh's starting rotation. He made his major league debut in the first game of a doubleheader on August 6, allowing just five hits and one run in a complete game, 10–1 victory over the Philadelphia Phillies. On September 6, he held the New York Mets to three hits in a complete game, 7–1 victory. In 12 starts with Pittsburgh, he had a 4–4 record, a 2.55 ERA, 42 strikeouts, 12 walks, and 62 hits allowed in innings pitched. The Pirates won the National League (NL) East Division and faced the Cincinnati Reds in the NL Championship Series (NLCS), but Tomlin made no appearances in the series as Cincinnati defeated Pittsburgh four games to two.