Warren Lahr signed with the Cleveland Browns in 1948 following his collegiate tenure at Western Reserve. Initially, he joined Paul Brown's team as a backup quarterback. A preseason knee injury sidelined him for the entire rookie year, thereby endangering his position on the roster. Undeterred, Lahr perservered and transitioned to a defensive back position, securing a starting role in the secondary during the 1949 season. He promptly made a significant impact, contributing to Cleveland's culmination of an undefeated campaign by winning the final AAFC Championship before the league's integration with the NFL.

Once positioned on defense, Lahr developed into a fundamental element of Paul Brown’s secondary. Renowned for his sharp instincts, robust tackling, and exceptional ball-hawking skills, he consistently nullified opposing wide receivers and maximized opportunities for turnovers. His impenetrable coverage on the perimeter was instrumental in Cleveland's smooth transition to the NFL, beginning with his role as cornerback during the celebrated 1950 NFL Championship victory against the Los Angeles Rams.

Lahr’s individual performance reached its peak nationally in the early 1950s. He received his sole Pro Bowl selection in 1953, a season during which he intercepted five passes and returned two for touchdowns. Over the subsequent two years, Lahr served as a key pass defender on the consecutive 1954 and 1955 NFL Championship teams, thereby solidifying his reputation as a performer in significant games. In total, he participated in seven NFL Championship Games, frequently assigned to cover the opposing team’s primary receiving threat.

His propensity for stepping up in high-stakes moments earned him legendary status in Northeast Ohio, none more famous than his five career postseason interceptions—including a pick-six in the 1954 title game blowout of Detroit.  He played for Cleveland until her retired after the 1959 season.

In his 11 years as a Brown, Lahr had 44 interceptions, returning them for 530 yards and five touchdowns.  He was inducted into the Cleveland Browns Legends in 2008.