Petronas was a brother of Empress Theodora and hence brother-in-law of Emperor Theophilos, under whom he advanced to the high court rank of patrikios and the post of commander of the Vigla guard regiment.

After Theophilos' death, he played a role in the ending of Iconoclasm, but was sidelined along with his brother Bardas during the minority of his nephew, Michael III, when power was held by the regent Theoktistos.

[3] In 842, as Theophilos lay dying, Petronas and the eunuch Theoktistos carried out the execution of the patrikios Theophobos,[3][2] a former Khurramite convert and general, whose troops had rebelled and proclaimed him emperor at Sinope some years before.

[4] Despite his kinship with Theophilos, the tale is told that the Emperor once had Petronas stripped naked and flogged in public because he had built a palace that overshadowed the house of a widow, in contravention of the law.

[6][7] Soon after that, Petronas and Bardas were successfully sidelined by the logothetes Theoktistos, while Niketiates was killed in an expedition against the Cretan Saracens, leaving the eunuch minister the dominant figure throughout Theodora's regency.

Petronas was placed in charge of all Byzantine troops assembling to confront the invasion, and through a brilliant coordination effort, three separate forces managed to converge on the Arab army, encircle it, and destroy it at the Battle of Lalakaon on September 3, 863.