Muhmenthaler was brought up in Grenchen and started his football with the youth teams of the local football club FC Grenchen. He also played in their first team in the Nationalliga B and they won promotion to the Nationalliga A in the 1970–71 season. The team also won the Uhrencup that year and Muhmenthaler scored the first goal in the final, as they beat FC Basel 3–1. In the summer of 1972, Muhmenthaler transferred to BSC Young Boys and he played there as Striker for three years. With the Young Boys, Muhmenthaler won the Uhrencup 1973 and he scored a goal in the semifinal against his previous club Grenchen.