Lagat first represented his native Kenya at 2000 Summer Olympics in Sydney, Australia. Using team tactics, Lagat and Kenyan team mate Noah Ngeny kept heavy favorite Hicham El Guerrouj from winning gold. In a very close finish, Ngeny took gold, El Guerrouj won Silver, and Lagat captured bronze. Lagat finished the year being ranked #3 in the world at 1500m.
The next summer, Lagat won the silver medal, behind El Guerrouj, at the 2001 World Championships in Athletics, and later that summer, at a meeting in Brussels Lagat set the Kenyan National Record and became the second fastest individual ever at 1500m when he ran 3:26.34, finishing second in this race behind Hicham El Guerrouj (3:26.13), in what was a failed attempt to break El Guerrouj's own world record of 3:26.00. Lagat finished the year being ranked #2 in the world at 1500m.
Lagat spent most of 2002 and 2003 chasing El Guerrouj. At the 2003 IAAF World Indoor Championships he earned the silver medal at 1500m, 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. His B sample test came back negative, clearing him of any charges. He was ranked 2nd and 4th in the world at 1500m in 2002 and 2003 respectively.
In the 3000m at the 2004 IAAF World Indoor Championships Lagat won his first international gold medal. Lagat was clearly overjoyed at his comeback since 2003. Throughout the spring he competed fiercely and beat his rival El Guerrouj in Zurich with a world leading time at 1500m of 3:27.40. At the 2004 Summer Olympics Lagat seemed poised to once again defeat El Guerrouj. The final was a very dramatic race, with Lagat and El Guerrouj battling down the final 100m, swapping the lead multiple times. It was El Guerrouj that prevailed, with Lagat close behind, earning the silver medal, running the final lap in under 52 seconds. He was however ranked #1 in the world at 1500m for the year.
The U.S. does allow dual citizenship and consequently races run by Lagat after May 7, 2004 could have been ratified as American records, since USATF rules only state that an athlete has to be a U.S. citizen competing in a sanctioned competition to be eligible to set a national record. However, at the 2005 USATF annual meeting, his 3:27.40 win in the 1500 meters, on August 6, 2004, in Zurich, was not ratified as an American record.
Nevertheless, 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 U.S. 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 1,500 m and 5,000 m 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.
In 2008, Lagat won both 1500 m and 5000 m runs at the US Olympic Trials in Eugene, Oregon, qualifying himself to compete with Team USA at the 2008 Summer Olympics in Beijing, China. Hopes were high that his success in both these events would continue at the Olympics. However, Lagat failed to advance beyond the semi-finals in the 1500m run. He was more successful in the 5000 m run, winning his semi-final heat to advance, but ultimately did not medal, running to ninth-place finish in the finals.
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. He stated that his placings at the 2008 Beijing Games were "the biggest disappointment in my athletics career". The following season, he began with high altitude training in Flagstaff, Arizona. After tying Eamonn Coghlan's record of seven wins in the Wanamaker Mile at the 2009 Millrose Games in New York, Lagat broke Coghlan's record with an 8th win at Millrose in 2010. As the reigning world champion in the 1500 and 5000 meters, he automatically received qualification in the events at the 2009 World Championships in Athletics and set his sights on retaining his titles. Despite being much older than some of his competitors, Lagat's desire to win had not faded and he said "I am more motivated than ever to go to the podium in Berlin". Lagat ultimately won the bronze medal in the 1500m and the silver medal in the 5000 m.
On February 6, 2010, Lagat set a new American record in his debut of the indoor 5000 meters at the Reebok Boston Indoor Games with a time of 13:11.50, beating previous record holder Galen Rupp who had set the record at 13:18.12 in 2009. On June 4, 2010, Lagat broke the American Record for 5000m with 12:54.12 at the Bislett Games in Oslo. Yet another area record came at the Rieti IAAF Grand Prix in August: chasing down Tariku Bekele in the 3000 m, he took second place with an American record of 7:29.00 – his first sub-7:30 minute time.
In July he broke his own 5000 m American record at the Monaco Diamond League meet by running 12:53.60 to finish a close second behind Mo Farah who set the British 5000m record in this race in a time of 12:53.11. At the 2011 World Championships in Athletics, the 5000 m final came down to a sprint finish and Lagat was beaten in the last straight by Mo Farah, leaving Lagat with his second consecutive silver medal at the world event.