Pătrășcanu was born in Bacău to a leading political family, as the son of Poporanist figure Dimitrie D. Pătrășcanu (Lucrețiu's mother, Lucreția, was a scion of the Stoika family of Transylvanian petty nobility). He became a Poporanist and later a socialist in his youth, joining the Socialist Party of Romania in 1919, and working as editor of its newspaper, "Socialismul" (1921). Professionally, he was educated at the University of Bucharest's Faculty of Law, from which he graduated in 1922, and at Leipzig University, earning his Ph.D. in 1925.