He was born in a houseboat on the Rotte and he grew up in the working class neighborhood of Rotterdam Crooswijk . [2 ] Although he wanted to be since childhood footballer he was fighting in the forties as an amateur his first parties. [4 ] His younger brother Aad Jansen was a professional boxer in 1974 and Dutch champion in the welterweight class. 
Leen Jansen began his professional career after he competed in the 1952 Summer Olympics in Helsinki . He reached the quarterfinals, where he faces Floyd Patterson excavated the worst of it.  In the fifties and sixties moisture Jansen total of 71 lots, of which he lost 9.  He lost his first professional game on July 11, 1956 from Franco Festucci.  Jansen became Dutch champion three times.
After his career as a boxer, he was a trainer. He had a boxing gym in the Chain Street in Kralingen , but it burned down in January 1982 on.  He later lived in St. Maartensdijk , where he began a new boxing gym.  Halil Ibrahim Özpamuk photographed Jansen for an exhibition on Rotterdam boxers , which was featured in the summer of 2012 at the Center for Visual Arts Rotterdam.  He also made an episode about the then 82-year-old former boxer for the television Vals plat of NTR . This episode aired on December 27, 2012. 
- 1952: Quarterfinals Summer Olympics
- April 26, 1954: Dutch middleweight champion 
- May 1955: Dutch middleweight champion 
- May 11, 1959: Dutch light heavyweight champion