1991 was Tomlin's first full season as a starter in the major leagues. On July 15 and 21, he pitched back-to-back shutouts against the Houston Astros and the Reds. For his performance, he was named the NL Player of the Week for the week of July 15-21. On August 6, he limited the Mets to four hits and one run in a complete game, 3–1 victory. At that point, he had an 8–4 record with a 2.35 ERA, but he was winless for the rest of the season, posting a 4.31 ERA. In the first game of a doubleheader against the Mets on September 26, he had a shutout going until the ninth, in which he gave up two runs, allowing New York to tie the game. However, Pittsburgh still won in 15 innings, 4–3. In 31 games (27 starts), he had an 8–7 record, 104 strikeouts, 54 walks, and 170 hits allowed in 175 innings. His 2.98 ERA ranked eighth in the league, and the Pirates won the NL East for the second year in a row. Jay LeBlanc of "The Washington Times" wrote that Tomlin is "best remembered for his clutch performance in Game 4 of the 1991 NLCS." With the Pirates down two games to one in the series, Tomlin held the Atlanta Braves to two runs over six innings in Game 4. He got a no decision, but Pittsburgh won the game 3–2 in 10 innings before ultimately losing the series in seven games.