Timlin spent the 1991 season with Toronto. He made his first major league appearance on opening day, April 8, pitching innings in relief against the Boston Red Sox. Two days later, he recorded his first strikeout (Tom Brunansky) and had his first win, after pitching an inning in relief against the Red Sox. For the regular season, Timlin appeared in 63 games, all but three in relief, compiling a record of 11–6 with three saves and a 3.16 earned run average (ERA). In the postseason, he made four relief appearances in the American League Championship Series (ALCS) against the Minnesota Twins, including taking the loss in Game 3 after giving up a home run to Mike Pagliarulo in the 10th inning. Timlin was sixth in Rookie of the Year voting.