A St. Louis County League star for 27 years, Mueller was a right-handed hitting third baseman who hit over .400 in five different seasons. He had eight hits in a double-header and on another occasion, had six hits in six at-bats in a single game. Les split his active career between two outstanding St. Louis County League teams: Olivette and Giesler-Jorgan, and played on six St. Louis County League Champions. In 1969 he was with Giesler-Jorgan when they won the Missouri State Amateur Championship. Mueller also managed the Creve Coeur American Legion team for seven years.