Despite an ailing Achilles tendon, the runner set a course record at 2:05:36.
Mosop, who was running his second marathon ever, took advantage of a move at mile marker 19 by 28-year-old Kenyan Wesley Korir, who made things interesting when he pushed into the lead during a water stop.
Korir couldn't maintain his pace and Mosop surged forward to claim the strong position.
He then put up the fastest mile of the race for mile 21 -- 4:40 -- and left the competition looking at his heels. he wins $100,000 for winning, and $50,000 for setting the course record.
Mosop's victory should be a disappointment for American Ryan Hall, who many thought would use the Chicago course as his coming out party.
USA's only real distance threat made changes to his practice routine and looked forward to taking advantage of an Achilles tendon injury to competitor Moses Mosop of Kenya.
But Hall couldn't keep up with the blistering pace and peeled off the leader's group at around mile 13.
Hall took fifth with a 2:08:04 time. He followed Korir (2:06:16), and Bernard Kipyego (2:06:29)