Laelianus declared himself emperor at Mainz in AD 268. Although his exact position is unknown, he is believed to have been a senior officer under Postumus. Laelianus represented a strong danger to Postumus because of the two legions he commanded. Despite this, his rebellion lasted only about two months before he was executed (reputedly by his own soldiers). The siege of Mainz was also fatal for Postumus; it is said he was slain when he refused to allow his troops to plunder the city following its capture.
Little is known about Laelianus. He shares the same nomen as a prominent Spanish noble family, the Ulpii, that included Trajan among its members, and may have been a relative. This is supported by the strong allusion to Spain on an aureus he struck, which featured the design of Hispania reclining with a rabbit to her side. If he indeed was a relative, this may be the reason Spain allied itself with Claudius II, after the death of Postumus, seemingly without a struggle.