He is based in Japan and trains with PACE Sports Management. He moved to Japan as a teenager on an athletic scholarship and specialised in long-distance track running, including the 5000 metres and 10,000 metres. He competed for, and graduated from, Ryutsu Keizai University.
At the 2005 Sapporo Half Marathon, he led for the first half of the race but had faded to fourth by the finish point. At the end of the year he represented Kenyan in the International Chiba Ekiden. Running the marathon distance with a relay team of Martin Mathathi, Daniel Mwangi, Mekubo Mogusu, Onesmus Nyerere and John Kariuki, he helped set a world record time of 1:57:06 hours for the popular Japanese event. Ndambiri won a second Chiba Ekiden title with Kenya the following year, where he won his 5-kilometre leg by a margin of over twenty seconds.
He won the 10,000 metres at the Kenyan Championships in 2007 and in his major international debut he finished fifth in the event at the 2007 World Championships. A month after his world appearance, he took a 5000/10,000 m double at the Japan Corporate Team Track & Field Championships. He attempted to gain selection for the 2008 Summer Olympics, but ended the trials in seventh and was not picked for the squad. Representing the Komori Corporation, he ran in the New Year Ekiden and won his 8.3 km stage. In the 2009 track season he became the first athlete to run the 10,000 m under 27 minutes on Japanese soil, taking the win at the Shizuoka International in an all-comers record of 26:57.36 minutes.
He came seventh at the RAK Half Marathon in 2010, setting a personal best of 1:01:07 hours. The following year he came fourth over 10,000 m at the Hyogo Relays and then claimed third place at the Sapporo Half Marathon in June. He made his debut over the marathon distance in December 2011 and made a successful transition by taking the Fukuoka Marathon title in a time of 2:07:36 hours.
- 1500 metres - 3:38.72 min (2004)
- 3000 metres - 7:42.98 min (2006)
- 5000 metres - 13:05.33 min (2005)
- 10,000 metres - 26:57.36 min (2009)
- Half marathon - 1:01:07 hrs (2010)
- Marathon - 2:07:36 hrs (2011)
- Nakamura, Ken (3 July 2005). "Ndereba and Mogusu victorious at Sapporo Half Marathon". 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 (23 November 2006). "Kenya dominate at Chiba Ekiden". IAAF. Retrieved 30 April 2016.
- Josephat Ndambiri Archived 2011-07-23 at the Wayback Machine.. PACE. Retrieved 5 July 2011.
- Nakamura, Ken (23 September 2007). "Kenyans are the double – Japan Corporate Team Track & Field Champs". IAAF. Retrieved 30 April 2016.
- Njenga, Peter (5 July 2008). "Kenyan Beijing squad announced as trials conclude in Nairobi". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (1 January 2009). "Fujitsu takes narrow one second victory in New Year Ekiden". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (6 May 2009). "National women's 200m record and world season 10,000m lead – Japanese competition round-up". IAAF. Archived from the original on 29 June 2011. Retrieved 4 December 2011.
- Nakamura, Ken (3 July 2011). "Njui and Kiplagat win in Sapporo". IAAF. Retrieved 30 April 2016.
- Nakamura, Ken (4 November 2011). "Running in his debut, Ndambiri triumphs in Fukuoka". IAAF. Retrieved 30 April 2016.