Leon Koen was born to a Sephardi Jewish family of merchants Aaron and mother Sarah, in Belgrade, Principality of Serbia.

[4] In 1896 he was awarded the silver medal for painting "Joseph's Dream" by the Munich Academy of Fine Arts.

[2] Koen survived a nervous breakdown in 1902 and was placed in a mental hospital in Belgrade, where he was under constant psychiatric supervision until his death.

Koen's close friend, David Pijade, collected his paintings for a major retrospective that took place in 1926 at King Peter's School near St. Michael's Cathedral in Belgrade.

[6] One year after Koen's death a grand retrospective was held at the Cvijeta Zuzorić Art Pavilion in Kalemegdan.