Lagat, now a Kenyan American, was born in Kaptel village, near Kapsabet town in Nandi District. His date of birth coincides with Jamhuri Day, the independence day of Kenya. He is a Nandi, sub-tribe of the Kalenjin people. He graduated from the Kaptel High School in 1994, where he had started his athletics career.
He joined Jomo Kenyatta University of Agriculture and Technology (JKUAT) in Nairobi 1996. Later in the same year he moved to Washington State University. In 2000, Lagat graduated from Washington State University –as athletic greats Henry Rono and Michael Kosgei from Kenya had done before him– with a degree in management information systems.
Lagat’s older sister Mary Chepkemboi is also a runner and African Champion from 1994.
In March, 2005, Lagat announced that he had become a naturalized citizen of the United States since May 7, 2004, despite competing for Kenya in the 2004 Summer Olympics. Since Kenya does not allow dual citizenship his silver medal in the 1500m is at stake, depending on how Kenya interprets its own laws (as of May, 2005, no additional news has arisen on this issue). Due to this switch of nationality, Lagat served a ban from international championship events. For this reason he missed the IAAF World Championships in Helsinki. A similar switch of nationality, in this case Kenyan to Danish, caused Wilson Kipketer to miss the 1996 Olympic Games.
Lagat owns three American records from races he had run in 2005 that were ratified by USATF. His first American records came indoors, with a 3:49.89 mile at Fayetteville, Arkansas, on February 11, 2005, during which his 1500 meters split time of 3:33.34 also established another new US record, en route to a win in the event. The performance replaced records by Steve Scott, who set the previous American indoor mile record of 3:51.8 in 1981, and the previous American 1500 meter indoor record held by Jeff Atkinson, who ran 3:38.12 in 1989. Lagat’s winning time of 3:29.40 at Rieti, Italy, on August 28, 2005, in the outdoor 1500 meters was ratified as his third new American record, improving upon the old record of 3:29.77, set by Sydney Maree in 1985.
At the 2007 World Championships in Osaka Lagat surpassed all his previous achievements by becoming the first athlete to become world champion in both the 1,500m and 5,000m at the same IAAF World Outdoor Championships. Similar feats were accomplished by Hicham El Guerrouj at the 2004 Olympics and Paavo Nurmi at the 1924 Olympics