Lewis started playing baseball around the age of 12, when he played baseball for the Junior American Legion. He batted left-handed and threw right-handed. In 1934, Lewis was star third baseman of a team for Post 23 that went to the Legion's version of the World Series. Recognized for his efforts, he was offered a tryout with the New York Giants, but manager Bill Terry decided against keeping Lewis on the team. A recommendation by a Legion World Series viewer led him to Joe Engel, who owned the Class A Chattanooga Lookouts, the Washington Senators' farm club at the time. Lewis started at third base late in the 1934 season, going 7-for-21 in 10 games. The following year, he went .303 with 40 doubles. He was called up by the Senators in September. Lewis began his major league career on September 16, 1935 by pinch hitting for pitcher Bump Hadley in the bottom of the 5th inning, where in his one batting appearance against Sad Sam Jones, he popped out before being replaced by Jack Russell. In eight games, he batted .107 with three hits on 28 at-bats, striking out five times. For 1936, Lewis won the third baseman position, although manager Bucky Harris held him out of the opening series of the season. Afterwards, he proceeded to hit successfully in his first six games. In 143 games, he had 175 hits, 100 runs, 21 doubles, six home runs, 67 RBIs with a .291 batting average, a .347 on-base percentage (OBP) and a .746 OPS, with a near even walk-strikeout ratio of 47-46. In the 139 games he appeared as a third baseman, Lewis logged in 1,214 innings, garnering a .933 fielding percentage with 152 putouts, 297 assists, 32 errors, and 24 double plays. While he led the league in errors, he finished in the top five for the other three defensive categories. The following year, he appeared in 156 games, having 668 at-bats and 210 hits (with both being career highs) along with 107 runs, a league high 162 singles, 32 doubles, ten home runs, 79 RBIs with a .314 batting average, a .367 OBP, a .792 OPS to go with 52 walks and 44 strikeouts. He received votes for Most Valuable Player, finishing 16th. He logged in 1,382 innings at third base, having a .938 fielding percentage while committing 146 putouts, 293 assists, 29 errors, and 32 double plays, all ranking in the top four for the league.