The 26-year-old Kosgei, who is an upstart in marathon running, said he broke away from the pack 200 metres to the finish to claim victory and pocketed $18,000 Sunday, reports Xinhua.
"I don't even have a manager and I have never run a major race before. I hope my win in Nairobi will accord me the opportunity of making it big like past winners," Kosgei told reporters.
Over 17,000 runners took part in the race, which also included the half marathon
While Elisha Barno claimed second position with a timing of two hours and 12:28 minutes, the third place went to Kirui Weldon.
In the women's category, Eunice Jeptoo clinched the title by finishing the 42km race in two hours and 43:02 minutes.
Hyline Kwambai won the second position with a timing of two hours and 44:15 minutes, which helped her finish 16 seconds ahead of third-placed Janet Jeruto.
Over 17,000 runners took part in the race, which also included the half marathon, 10km, and wheelchair categories for both men and women.