Bahram I's reign marked the end of the Sasanian tolerance towards Manichaeism, and in 274, with the support of the influential Zoroastrian priest Kartir, he had Mani imprisoned and executed.

Although the oldest of Shapur's sons, Bahram I was ranked below his brothers, probably due to his mother's lowly origin: she was either a minor queen or a concubine.

[5][6] During Shapur's reign, Bahram I served as the governor of the newly conquered region of Gilan, situated on the southwest shore of the Caspian Sea.

He is mentioned in an inscription on the wall of the Ka'ba-ye Zartosht at Naqsh-e Rostam near Persepolis in southern Iran, which Shapur I had created in order to praise his sons by citing their names and titles.

[9] He then made a settlement with Narseh, who agreed to give up his entitlement to the throne in return for the governorship of the important frontier province of Armenia, which was constantly the source of war between the Roman and Sasanian Empires.

[16] The front of Bahram I's coins shows him wearing the distinctive crown of the angelic divinity Mithra; a headgear decorated with ray-shaped spikes.

[17] The lost Book of the Portraits of Sasanian Kings portrayed Bahram I as "standing, holding a lance in the right hand and leaning upon a sword held in the left, and wearing a red gown and trousers and a gold crown topped with a sky-blue globe".

It displayed him on horseback, accepting the diadem of kingship from the Zoroastrian supreme god Ahura Mazda, who is also depicted sitting on a horse.