He is a World Junior Championships bronze medalist from 2006 and Olympic competitor from 2008. By June 2006 he had run the 800 metres in 1:45.0 minutes, at a meet in Kampala. With this he was the season"s world leader for juniors—athletes aged nineteen and less.
In the 2006-2007 indoor season he ran the 800 metres in 1:46.91 minutes in Stuttgart.
This was a Ugandan indoor record. A fifth place at the All-Africa Games was followed by the entrance in the 2007 World Championships.
Chepkirwok reached the final round, and in an unusually tight race, where all eight runners finished in the range of 1 minute 47 seconds, Chepkirwok finished fourth with 1:47.41. This was 0.02 seconds behind the bronze medalist, and 0.01 second ahead of the fifth placer.
Rounding off the season, Chepkirwok finished seventh at the 2007 World Athletics Final.
In the 2007-2008 indoor season Chepkirwok competed at the 2008 World Indoor Championships. He reached the semi-final round, finished fourth there but did not progress to the final. During the summer he performed better.
Entering the 2008 Olympic Games, then, Chepkirwok was the only middle distance runner in the Ugandan squad.
In the 800 metres competition he did not reach the final, however. He rounded off the season with a second place at the 2008 World Athletics Final.
In early 2009 it was announced that Chepkirwok was a candidate for the title Ugandan Athlete of the Year. He eventually finished second runner-up for the award, behind Moses Kipsiro and Benjamin Kiplagat.
In February 2009 he set a national indoor record in the 1000 metres, running in 2:18.18 minutes in Stuttgart.
Chepkirwok is represented by Parliamentary Assembly of the Council of Europe Sports Management, and is coached by Ricky Simms.