He is the American record holder in the 1500 m and mile run indoors, as well as the 1500 m, 3000 m, and 5000 m outdoors, and is the Kenyan record holder at 1500 m outdoors. Lagat is the second fastest 1500 m runner of all time, behind Hicham El Guerrouj.

Lagat is a five-time Olympian, having competed in the 2000, 2004, 2008, 2012 and 2016 games, and is a thirteen-time medalist in World Championships and Olympics including five gold medals.