Jaeger became a mountain guide in 1975. On 15 October 1978, he participated in the first French ascent of Mount Everest. The team leader was Pierre Mazeaud. Other team members included Jean Afanassieff and Kurt Diemberger. Jaeger and Afanassieff were the first to climb and then ski down one of the 14 mountains on Earth higher than . From 27 July to 27 September 1979, he spent 60 days alone at altitude on Huascarán to study the effects of super-acclimatisation on himself. He published an account of his experience in Carnets de Solitude the same year.