Geijerstam was born in 1946. His career in Swedish show business started with sound engineering in the early 1960s. In 1966 he became a member of the Swedish pop band Ola & The Janglers, one of the major rivals to Benny Andersson’s band The Hep Stars.
In 1973 Claes was a member of the duo act Nova who competed against ABBA in the Swedish selections for the Eurovision Song Contest. Nova’s contribution, You Are Summer, You Never Tell Me No, won the selections. ABBA’s Ring Ring only finished third.
Claes joined ABBA as a sound engineer on their 1974/75 tour of Europe, and the tour of Europe and Australia in 1977. He was also there to take care of the sound on the 1979/1980 tours of North America, Europe and Japan. As a performer he actually toured places like the United States before ABBA: he was there with his group Rocket as early as the mid-1970s.
Claes af Geijerstam became a television personality in the early 1970s but is best-known in Sweden as a popular DJ on radio and in discotheques. His DJ-ing (in Swedish) and music is now available around the clock via his own app that carries his nickname, CLABBE. In more recent years, he has served for a few years as one of the judges on the Swedish version of Pop Idol. In 2016, he hosted the 50th anniversary party for Benny and Björn as composers.