In 1952, he began working as a designer for the Swedish furniture company OPE Möbler AB in Ängelholm. During his time at OPE, he designed a number of modern pieces of furniture that went into series production, including chairs, tables and cabinets.
In 1955, Kienzle returned to Germany and worked for the furniture company Horgenglarus AG in Glarus, Switzerland. He was appointed art director of the company in 1958 and designed many modern pieces of furniture for Horgenglarus, including the STH11 chair, which has become a design classic.
In addition to his work for OPE and Horgenglarus, Kienzle also worked as a freelance designer for other furniture companies, such as Wilkhahn and Wössner.
Wilhelm Kienzle is considered one of the most important post-war designers in Germany and is particularly known for his modern, functional furniture.