Garrett made his major league debut at second base, and played most of his first month in the majors there. Shortly afterwards, the 21 year-old lefty hitter settled into a lefty-righty platoon with Ed Charles at third base. He batted just .218 with one home run his rookie season, however, he would match that home run total in the 1969 National League Championship Series against his former franchise. With the Atlanta Braves leading 4–3 in the fifth inning of game three, Garrett hit a go-ahead two run home run to seal the three-game sweep for the Mets. For the NLCS, Garrett batted .385 with three runs batted in and three runs scored. He struck out in his only official at-bat in the World Series.