Ernst Reuter was born on July 29, 1889, in Apenrade, a small town near Flensburg in Northern Schleswig, which belongs to Denmark today but at the time was part of the Kingdom of Prussia. Reuter came from a middle class, protestant background - his father Wilhelm taught at the Royal Prussian School for Navigation, his mother Karoline was a pastor's daughter. In 1892 the family moved to the town of Leer.
Ernst Reuter was educated according to the classical-humanistic ideal of the bourgeoisie. After he earned his diploma with a university entrance qualification, he took up studies in History, German, and Geography at the University of Marburg in 1907, transferring two years later to the Ludwig-Maximilians-University in Munich for a short time.
It was during his time as a student that he first encountered the ideas of socialism which influenced him immensely. Against his parents' wishes he did not choose a career in the civil service, which he was qualified for after successfully passing the state examination, but made his way through the working-class-movement into social democracy. Consequently, in 1912 he became a member of the SPD, the Social Democratic Party of Germany. After working as a private tutor, journalist, and itinerant speaker, he moved to Berlin in August 1913.