He left school at the age of 15. He signed up to join an expedition on a warship, with the aim of becoming a naval officer, but illness prevented them from achieving this goal. Clemmensen studied at the Copenhagen Polytechnic Institute (now the Technical University of Denmark). He emigrated to the United States in 1900 and worked in the pharmaceutical industry. He joined the pharmaceutical company Parke, Davis & Co in Detroit, Michigan. For the invention of the Clemmensen reduction, he received his Ph.D. in 1913 from the University of Copenhagen.