The AAFC dissolved after the 1949 season and the Browns were absorbed by the NFL. Lahr stayed with the team after the transition and was a mainstay of the defensive backfield throughout the 1950s, playing opposite Tommy James. The Browns won the NFL championship in their first year in the league, when Lahr recorded eight interceptions, including two he ran back for touchdowns. He had two interceptions in the Browns' 30–28 win over the Los Angeles Rams in the championship game.