He started cycling in 1961 and qualified in 1963 for what was then the A-class for amateurs in Switzerland. Pfenninger started for the RV Höngg club. Louis Pfenninger began his professional career at the age of 20 with the Swiss Filotex team . He celebrated his first major success in 1968 at the six-day race in Montreal as a duo with his namesake Fritz Pfenninger . In the same year he managed to win the Tour de Suisse . In 1970 he became Swiss champion in the individual time trial and a year later in the road race. He also won the six-day race in Zurich in 1971 together with Klaus Bugdahl and Dieter Kemper . Pfenninger won the Tour de Suisse again in 1972 with a lead of 21 seconds over Roger Pingeon . Since he took part in the Tour de Suisse eleven times (and finished it ten times), he was also called Monsieur Tour de Suisse . In 1975 he ended his career.