Simon was born approximately 35 CE at Gerea although some believe he was actually from Judea.
When the Jewish revolt against the Romans began in Jerusalem in 66 CE, Simon became one of the military leaders of the rebellion. With the various Jewish groups clashing with each other Simon was considered a radical who loathed the more moderate factions who sought to negotiate with the Romans.
Simon's first big success was during the first the year of the revolt: The Roman army led by Cestius Gallus was retreating from Jerusalem after their attempt to put down the revolt. At the Beth Horton Pass, bar Giora and his soldiers cut off the rear guard in a "crushing defeat." Most of the Roman troops were killed and Gallus barely escaped. Simon took the baggage train back to Jerusalem with the plunder from the Romans and was celebrated as a hero.
Rejected from leadership roles in Jerusalem because he was considered "too extreme" and a "radical" by the Jewish authorities who were trying to placate the Romans, and accused of engaging in "brigandage" against internal enemies, collaborators, and sympathizers of Rome, bar Giora left Jerusalem with his followers and tried to make himself the leader of Acrabatene, near Samaria. There he "attacked the wealthy, sacking their houses and molesting their persons" until the Jewish leadership in Jerusalem sent a force led by Hanan ben Hanan, an ex high priest to stop him. Simon escaped!
Simon and his followers fled to the mountaintop fortress at Masada where they partnered with the other group there, the Sicarii faction, on local raids. Eventually he left Masada and began to attract a group of followers and soon began his campaign to take Jerusalem from his chief rival, John of Gischala, head of the Zealots who now controlled the city.
Simon and his followers, growing in size with the inclusion of thousands of former slaves (as part of his message he had promised "liberty for the slaves") was able to overtake the Idumeans and the Hebron area. Then he attempted to take Jerusalem from The Zealots. The Zealots came out and attacked and were repulsed by bar Giora's troops, but fearing that he was not yet strong enough to capture the city Simon and his army fled.
Building up more strength and with the domination of Hebron where he found "vast stores of wheat," his forces now numbered upwards of 40,000 men. There they ravaged the land in order to live off of it.
The Zealots then kidnapped Simon's wife along with her attendants who were taken hostage. Distraught, Simon ruthlessly captured and mutilated anyone outside of the vicinity of Jerusalem who was deemed to have any connection with The Zealots. He also vowed, upon his capture of the city, to butcher the entire population if his wife was not returned. The Zealots fearing ben Giora's wrath relented and returned her.
With the help of his former enemies, the Iduemeans who were now in conflict with The Zealots, ben Giora was eventually summoned back into the city where he took control of the upper part of the city as well as portions of the lower section.
Skirmishes between the various Jewish sects continued until Titus, the Roman military leader, came to the gate at 70 CE to put down the revolt. At this point the various squabbling groups came together to fight against the Romans and, combined, the groups began to inflict serious damage against the Romans.
It was estimated that ben Giora had 10,000 men organized into fifty divisions under him, along with his partners the Idumeans who had 5,000 men. He also possessed artillery seized from the Roman garrison at the Antonio Fortress at the beginning of the siege.
At around this time, Titus in a display of sheer brutality sent the hands cut off from Jewish prisoners to the Jewish leaders exhorting them to surrender. The response was that they would rather die an honorable death than serve a live of slavery and that in any case they would be saved by God.
The city soon began to starve and in many cases its inhabitants resorted to cannibalism. Still when the Idumean leadership sought to surrender, bar Giora had their peace emissaries murdered and arrested the leaders.
Eventually the revolt was mostly put down and Herod's great temple (the Second Temple) had been set aflame and destroyed. After the other leadership had surrendered, bar Giora went into hiding in the subterranean passages of the city.
Soon, overcome with hunger, he emerged from his hiding place at the site of the Temple wearing a white shroud and a purple cloak. His appearance astonished the Roman soldiers. After identifying him as the rebel leader, he was taken prisoner.
Simon was taken to Rome as part of the great triumph celebrating the victory over the Jewish people. There he was executed: Some say he died either having being hurled from the Tarpeian Rock, which was a steep cliff off of the Capitoline Hill or was strangled at the Mamertine Prison.