According to Tacitus, before his service in the Praetorians, he distinguished himself with his bravery and skill in helping to subdue the mutiny on the Germanic frontier immediately after the death of Augustus.
Cassius was disturbed by the increasingly unbalanced emperor, and angered at Caligula's mocking of his voice and his supposed or real effeminacy. It is reported that whenever Caligula had Chaerea kiss his ring, Caligula would, according to Suetonius, "hold out his hand to kiss, forming and moving it in an obscene fashion." Cassius was also made to use degrading watch-words at night, including "Venus" (slang for a male eunuch) and "Priapus" (erection).
Unable to bear this any longer, Chaerea planned to assassinate Caligula during the Palatine games held in January. Cassius' plot was one of several that formed around the same time and eventually coalesced into one broad conspiracy including a number of Praetorians, Senators, and Equestrians. On January 24, Cassius struck, and Caligula was killed. At the same time, Caligula's wife Caesonia and daughter Julia Drusilla were murdered, completing the task of destroying the emperor's immediate family. Cassius was sympathetic to his fellow conspirators in the Senate, and wanted the destruction of the principate.
Unfortunately for Cassius, he did not control the loyalty of the majority of the Praetorians. The Praetorians proclaimed Caligula's uncle, Claudius, as emperor. Shortly afterwards, Cassius was sentenced to death, one of the few assassins to be actually condemned. Cassius requested to be executed with his own murder weapon, and this was granted.
Cassius Chaerea is portrayed very sympathetically in Robert Graves' I, Claudius novels. Here Cassius is portrayed as having had a long and distinguished career in service of Rome, including being the only surviving officer of the massacre at Teutoburg Forest. He later serves under Caligula, whom he protected in his youth. Cassius insists that he killed Caligula for the Republic's sake, and Claudius sympathizes with him. Cassius is foretold in the Sibyl's prophecy to be "the horse" that will kill Caligula, as Caligula rode on Cassius' shoulders as a child. As he did in fact according to Suetonius, the new Emperor Claudius decides he must have Cassius Chaerea executed, not so much for the murder of the insane Caligula, but for ordering the murder of Caligula's wife and infant child.
- Cornelius Tacitus. Anthony John Woodman, ed. The Annals. Indianapolis, Indiana: Hackett Publishing. p. 19. ISBN 978-0-87220-558-1. Retrieved 2011-01-24.
- "The Lives of the Twelve Caesars". University of Chicago Press. 2010. Retrieved January 23, 2010.