He was once an average guy, flying around saving people from bad bounty hunters or nasty criminals. One day, however, little Saren found a Holotape floating around in space. He picked it up with his tractor beam, put it in his holotape player, and this played:
(Note that no copies of the original remain, this is the attempt by MC Sovereign to indoctrinate A Big Lump, The Great Whinger, and Their Best Buddy, which failed.)
He was indoctrinated by the music, and proceeded to seek out MC Sovereign himself. He found him sleeping at the edges of Dark Space. After MC woke up, he offered to be Saren's personal flagship and they formed a partnership, determined to spread MC Sovereign's dominance after his falling out with DJ Harbinger. Saren gradually went more and more insane as he travelled aboard Sovereign. He recruited the Heretic Geth and rogue Krogan and marched around the Galaxy, invading planets and searching for the Conduit. Seen here enjoying a toilet break on a R'tas Brand Toliet Deluxe-Uber Edition, Saren rages after being interrputed:
Shortly after this event, Saren proceeded to land on Virmire, a planet on the borders of League Space, and attack the Normandy riding on his big hoverboard. He was knocked off his hoverboard so he raged again and got puched in the face, so he fixed his hoverboard and flew away on it.
He then assisted MC by heading to Ilos, a planet which contained the Conduit, and taking the back door into The Citadel. He then opened the arms of the station to let MC in. After MC started to jack the Broadcast Tower, Saren was confronted by the Normandy Crew. After being convinced that he was being controlled to carry out MC Sovereign's every command, he shot himself to end the torment, falling epicly through the glass floor while dramatic music played. However, MC took control of and reanimated him using his implants, turning him into an evil frog. The evil frog then got killed, disabling MC's shields and allowing him to be brought down. Poor old Saren, eh? A victim from the very start.