Marcel Janco was born on May 24, 1895 in Bucharest, Romania, to a wealthy Jewish family. From a young age, he felt guilty about his wealthy lifestyle and developed a desire for social justice.
His father, Hermann Zui Iancu, was a textile merchant. His mother, Rachel née Iuster, was from Moldavia. The couple lived outside Bucharest's Jewish quarter, on Decebal Street. He was the oldest of four children. His brothers were Iuliu (Jules) and George. His sister, Lucia, was born in 1900.