Prior to his change of domicile to the US, Lagat had an extensive competitive career representing his native country.

With his personal best of 3:26.34, Lagat is the Kenyan record holder at 1500 m outdoors, and is the second fastest 1500 m runner of all time, behind Hicham El Guerrouj.

Later in the same year he moved to Washington State University where other Kenyan athletic greats such as Henry Rono and Michael Kosgei prospered.

Lagat's siblings have also competed in athletics; his older sister Mary Chepkemboi won the 3000 m at the 1984 African Championships in Athletics,[7] his younger sister Viola Cheptoo placed second in the 2021 New York City Marathon[8] Lagat first represented his native Kenya at the 2000 Summer Olympics in Sydney, Australia.

[9] In the 1500 m final, Morocco used team tactics to try to ensure a gold medal for heavy favorite Hicham El Guerrouj.

In a very close finish, Noah Ngeny took gold, El Guerrouj won silver, and Lagat captured bronze.

Lagat finished the year being ranked #3 in the world at 1500 m. The next summer, Lagat won the silver medal, behind El Guerrouj, at the 2001 World Championships in Athletics, and later that summer, at a meet in Brussels, Lagat set the Kenyan National Record and became the second fastest individual ever at 1500 m when he ran 3:26.34, finishing second in this race behind Hicham El Guerrouj (3:26.13), in an attempt to break El Guerrouj's world record of 3:26.00.

At the 2003 IAAF World Indoor Championships, he earned the silver medal at 1500 m, this time behind Driss Maazouzi of France.

Lagat withdrew from the 2003 world outdoor championships after a blood test showed traces of EPO in his system.

Throughout the spring he competed fiercely and beat his rival El Guerrouj in Zürich with a world leading time at 1500 m of 3:27.40.

El Guerrouj prevailed, with Lagat close behind, earning the silver medal, running the final lap in under 52 seconds.

[24] Lagat had concealed the fact that he was running with an injured Achilles tendon, a problem which hampered his training and contributed to his poor showing at the Olympics.

[32] At the 2012 Olympics in London, Lagat finished fourth in the 5000 m race, crossing the line 1.33 seconds behind the leader, Mo Farah, after being tripped up when he was going into his kick by Isiah Kiplangat Koech.

He entered the last lap in sixth place more than two and a half seconds behind race leader Galen Rupp.

Lagat ran his final race as a professional on September 3, 2016, at the ISTAF meet in Berlin where he came in second place in the 3,000 meters.

In preparation he had a 30 km training run with the world record holder Eliud Kipchoge in Kenya.

[50][51] Lagat lives in Tucson, Arizona and Tübingen, Germany with his wife Gladys Tom (a Canadian of Chinese descent),[52] whom he met while they both attended Washington State University,[53][54] and their son Miika Kimutai (born in 2006) and daughter Gianna (born 2008).