Słonimski was born in Warsaw in 1922 and he finished "underground" studies of medicine during World War II in occupied Poland.

In 1947, Słonimski emigrated and settled in France, as members of Armia Krajowa were prosecuted by the newly established communist government in Poland.

Once in Paris, he joined the group of Boris Ephrussi at Sorbonne University and started working in the field of genetics.

[2] He frequently hosted Polish intelectualists and dissidents, such as Adam Michnik, Tadeusz Mazowiecki, Maria and Leszek Kołakowski.

[1] He was one of the first to show that genetic information is passed outside of the nucleus in mitochondria,[4] and his subsequent, much cited work led to establishment of the field of mitochondrial genomics.