Marcus Opellius Macrinus was born in AD 164 in Caesarea, a harbour town in Mauretania. There are two stories surrounding his origins.
The latter is perhaps more likely. For when he moved to Rome, Macrinus gained a reputation as a lawyer. Such was the reputation he achieved that he became the legal adviser to Plautianus, the praetorian prefect of Septimius Severus, who died in AD 205. Thereafter Macrinus worked as director of traffic on the Via Flamina and then became financial administrator of Severus' private estates.
In AD 212 Caracalla made him praetorian prefect. In AD 216 Macrinus accompanied his emperor on campaign against the Parthians, and in AD 217, whilst still campaigning he received consular rank (consular status without office: ornamenta consularia).
Macrinus is described as a stern character. As a lawyer, though not being a great expert in law, he was conscientious and thorough. As praetorian prefect he is said to have had good judgement whenever he sought to act. But in private he is also reported to have been impossibly strict, frequently flogging his servants for the slightest of mistakes.
In the spring of AD 217 Macrinus intercepted a letter, either from Flavius Maternianus (commander of Rome in Caracalla's absence) or from an astrologer of Caracalla's, denouncing him as a possible traitor. If only to save his own life from the vengeance of the bloodthirsty emperor, Macrinus needed to act.
Though as Martialis tried to get away, he was himself killed Caracalla's mounted bodyguards. This meant there was no witness to link Macrinus with the murder. And so Macrinus feigned ignorance of the plot and pretended grief at his emperor's death.
Caracalla though had died without a son. Their was no obvious heir.
Macrinus though knew very well that his being emperor depended entirely on the goodwill of the army as he at first had no support at all in the senate. - He was the first emperor, not to be a senator !
The senate, faced with no alternative but to acknowledge Macrinus as emperor, though was in fact quite glad to do so, as the senators were simply relieved to see the end of the hated Caracalla. Macrinus won further senatorial sympathies by reversing some of Caracalla's taxes and announcing an amnesty for political exiles.
Meanwhile though Macrinus should win an enemy who shoudl seal his fate. Julia Domna, the wife of Septimius Severus and mother of Caracalla, quickly fell out with the new emperor. Most likely she came to know what part Macrinus had played in her son's death.
Meanwhile Macrinus was gradually losing the support of the army, as he tried to disentangle Rome from the war with Parthia which Caracalla had begun.
His position weakened by increasingly hostile military, Macrinus next had to face a revolt by Julia Maesa. Her fourteen year old grandson, Elagabalus, was hailed emperor by the Legio III 'Gallica' at Raphanaea in Phoenicia on 16 May AD 218.
As both Macrinus and his young challenger were in the east, there was no effect the powerful legions based at the Rhine and Danube could have. Macrinus at first sought to quickly crush the rebellion, by sending his praetorian prefect Ulpius Julianus with a strong cavalry force against them. But the cavalrymen simply killed their commander and joined the ranks of Elagabalus' army.
In an attempt to create the impression of stability, Macrinus now pronounced his nine year old son Diadumenianus joint Augustus. Macrinus used this as a means to cancel the previous pay reductions and distributing a large bonus to the soldiers, in the hope that may win back their favour. But it was all in vain. For soon after an entire legion deserted to the other side. So dire did the desertions and mutinies in his camp become that Macrinus was forced to retire to Antioch.
The governors of Phoenicia and Egypt remained loyal to him, but Macrinus cause was lost, as they could not provide him with any significant reinforcements. A considerable force under the command of the rival emperor's general Gannys finally marched against him. In a battle outside Antioch on 8 June AD 218 Macrinus was decisively defeated, abandoned by most of his troops.
Disguised as a member of the military police, having shaved his beard and hair, Macrinus fled and tried to make his way back to Rome. But at Chalcedon on the Bosporus a centurion recognized him and he was arrested.
Macrinus was taken back to Antioch and there he was put to death. He was 53. His son Diadumenianus was killed soon after.