Julianus was born to Quintus Petronius Didius Severus and Aemilia Clara. Julianus's father came from a prominent family in Mediolanum (Milan) and his mother was a North African woman of Roman descent. Clara came from a family of consular rank. His brothers were Didius Proculus and Didius Nummius Albinus. His date of birth is given as 30 January 133 by Cassius Dio and 2 February 137 by the Historia Augusta.
Didius Julianus was raised by Domitia Lucilla, mother of Roman Emperor Marcus Aurelius. With Domitia's help, he was appointed at a very early age to the vigintivirate, the first step towards public distinction. He married a Roman woman named Manlia Scantilla, and sometime around 153, Scantilla bore him a daughter and only child Didia Clara.