Marcus Opellius Macrinus was a man who, in more settled times might have made a solid, unspectacular emperor. He was honest, thorough, hard-working and a good administrator. However, in more settled times, Macrinus would never have become emperor at all, for by conventional standards he was completely unqualified for the job.
Macrinus was a provincial, born in the province of Mauretania Caesarienis (in modern Algeria). He had no family connection with the Antonine dynasty or the family of Septimius Severus, and certainly was no blood relative. Furthermore, he was not even a senator, which until Macrinus' succession was regarded as the minimum qualification for the position of emperor.
Instead of family or political connections, Macrinus managed to get to within striking position of the imperial throne by solid, hard work and reliability. There are various stories about the early years of Rome's 23rd emperor, including that he had worked as a courier and even as a gladiator. We can discount the latter as a later defamation, as does the historian Cassius Dio who was a contemporary of Macrinus and knew the man well by reputation; indeed he may well have met him personally.
It is probable that Macrinus' parents were well-respected but not very wealthy small-town grandees. They were able to fund their son's education as a lawyer (there is no record of Macrinus having any siblings). When the ambitious young man went to Rome the shortcomings of his provincial education were exposed. As Dio remarked, his knowledge of the law was less reliable than the punctiliousness with which he observed those laws.
In the snake-pit of Roman politics loyalty and reliability were every bit as desirable as talent, and consequently Macrinus became a protégé of the Praetorian Prefect of Septimius Severus. Loyal and reliable as he may have been, the country lad from Mauretania must have had some political ability for he not only survived his patron's fall from grace, but afterwards even managed to obtain a promotion. This promotion was to the post of manager of the vital Via Flaminia, one of the main road arteries of Italy. By this time Macrinus was married, to a woman of whom we know only the name, Nonia Celsa, and that from an unreliable source, the notoriously error-prone Historia Augusta.
Eventually Macrinus switched from imperial functionary to imperial courtier, holding a series of positions in the household of Septimius Severus. When Severus died, Macrinus was placed in charge of the finances of the new co-emperors, Geta and Caracalla. The political acumen of Macrinus ensured that he had no ties to the ill-fated Geta. Indeed, once Caracalla had murdered his brother in 211, Macrinus not only survived the subsequent purge but was promoted again, this time to the post of Praetorian Prefect.
Septimius Severus had decided that the office of Praetorian Prefect was too powerful for one man to hold alone, so Macrinus shared his duties with a colleague, the elderly Adventus. However, Macrinus was undoubtedly the senior of the two, and the man who most enjoyed Caracalla's trust. As a member of the emperor's advisory council Macrinus was privy to the innermost secrets and decisions of the imperial administration. As part of his duties Macrinus commanded the legion II Parthica. Since the Praetorian guard was now defunct, this legion represented the military power nearest Rome, and was another sign of Caracalla's trust in his subordinate.
This trust vanished during the Parthian campaign which took Caracalla and Macrinus east in 214. At some time in late 216, two different prophecies reached Caracalla that Macrinus would kill and replace him. Caracalla affected not to believe this, and indeed had one of the prophets executed. However, Macrinus noted that key subordinates loyal to him were transferred out of his control, as was command of II Parthica. Macrinus himself received personal awards such as the title of 'clarissimus' and the right to wear consular insignia, but the shrewd politician cannot have but noted that he was being isolated and his powers replaced by honours.
Consequently, when Caracalla wanted to go on a sightseeing trip to a famous local temple, it seems that Macrinus made sure that one Martialis was among the emperor's retainers. Martialis hated Caracalla, either because Caracalla had unjustly ordered his brother killed, or because Caracalla had denied Martialis promotion to the rank of centurion. When Caracalla dismounted and sought privacy to empty his bowels, Martialis approached and stabbed Caracalla with a concealed knife. The emperor's bodyguard promptly killed the assassin (and according to Cassuis Dio other retainers loyal to Macrinus quietly finished off Caracalla). In a quick, clean killing Rome's emperor was dead, and the obvious suspect was dead also.