Muhmenthaler was brought up in Grenchen and started his football with the youth teams of the localfootball clubFC 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. In 1975, Muhmenthaler transferred with a two-year contract to FC Basel. He made his team debut in the 5–1 home win against Grasshopper Club Zürich and Muhmenthaler scored three of the goals. He was part of the title winning team in the 1976–77 football season. During a cup match in February 1976, he suffered a meniscus rupture. Nevertheless, Basel extended his contract despite the fact that Muhmenthaler had to undergo three operations within the following two years. Muhmenthaler reached his third Uhrencup Final with his third club in 1977 but was unable to repeat the success, the final ended in a 6–1 defeat against Neuchâtel Xamax. Reaching the final again in the next year, Basel won the 1978 Uhrencup 2–1 against the same finalists. Muhmenthaler was forced to terminate his playing career early due to his injury.
Career as referee
After his time as an active player Muhmenthaler became referee in the Swiss Football League. His career in brief: 1980–81: 3. Liga. 1981 to 1984: 2. Liga. From the season 1984–85: referee in the Swiss Football Association. From 1989: FIFA-Referee. From 1980 until the end of his career in December 1997 he conducted about 250 Swiss and some 75 international games. Amongst these he describes as one of his personal highlights the 1998 FIFA World Cup qualification match on 11 November 1997 between Italy and Russia in front of 80 000 spectators at the Stadio San Paolo, Naples. As part of the then practiced German-Swiss referee exchange 1988–1990, Muhmenthaler led four games in the Bundesliga.
