[2] His father has been identified as Lucius Julius Marinus, attested as governor of Lower Moesia in January 97, and whose tenure as suffect consul has been dated around AD 93.
[5] Simplex began his career in the vigintiviri, as a member of the quattuorviri viarum curandarum, then served as military tribune in Legio IV Scythica, then stationed in Moesia.
As a quaestor, he was one of those allocated by lot to one of the ten public provinces administered by a proconsul; in his case, Simplex was assigned to Macedonia.
Simplex was then appointed by emperor Domitian to be legatus legionis or commander of Legio XI Claudia Pia Fidelis, which was stationed facing the Danube in Vindonissa.
Simplex is known to have married Julia Tertulla, the sister or daughter of Gaius Julius Cornutus Tertullus, the friend of Pliny the Younger.