He gained an athletic scholarship as a teenager to train and study in Japan. In 2003 he ran bests of 13:38.70 minutes and 28:13.34 minutes for the 5000 metres and 10,000 metres, respectively. He also ran at the Sapporo Half Marathon that year and came eighth in a personal best time of 1:03:16 hours. He improved his 5000 m best to 13:29.18 minutes in the 2004 season. He improved this further in 2005, running 13:18.21 minutes. He had two consecutive wins over the distance at the Oda Memorial in 2006 and 2007.
At the 2005 New Year Ekiden, he ran for the JAL AGS corporate team and won the first stage, completing the distance just one second short of Martin Mathathi's record. The 2005 International Chiba Ekiden in November saw Mwangi help break the world record for the ekiden marathon relay race as part of a Kenyan team including Josephat Ndambiri, Martin Mathathi, Mekubo Mogusu, Onesmus Nyerere and John Kariuki. He returned to the New Year Ekiden race in 2006 and again won the first leg for JAL AGS.
In 2008 he switched to marathon running and made his debut at the Beppu-Ōita Marathon. He fell over and lost a shoe mid-race but rejoined the leading pack and eventually finished in fifth place with a time of 2:14:28 hours. His second outing over the distance came at the Fukuoka Marathon and his run of 2:14:13 hours was a little faster, but this was only enough for twelfth place. He has not competed since this race.
- Mwangi, Daniel Muchunu. IAAF. Retrieved on 4 December 2011.
- Nakamura, Ken (6 July 2003). "Ndereba wins Sapporo Half Marathon in a course record". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (1 March 2007). "Sawano out-duels Walker in Shizuoka". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (1 January 2005). "Konica-Minolta win final stage battle in Maebashi". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (23 November 2005). "Kenyan men win in Chiba with World's fastest ever time". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (1 January 2006). "Konica-Minolta wins 50th New Year's Ekiden". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (3 February 2008). February 2008 AIMS Results. AIMS. Retrieved 4 December 2011.