In the first half of the 1930s Knud Jacobsen won some of the most important amateur cycling races in Denmark, including the gruelling Sjælland Rundt twice (1932, 1934). Jacobsen also had noted victories on the track, winning the team pursuit at the Nordic Championships in 1935 and 1936. Also in 1936 he competed in the road race at the Berlin Olympics where he finished the course, but his time was not recorded. That same year Jacobsen turned professional and competed in races over the next decade. He later earned a contract with the German team Wanderer, winning the 14th stage of the Tour of Germany in 1938.