She won the 2000 and 2005 10K races at the Ottawa Marathon, setting a course record in her second victory. In 2006, she moved to Japan to represent local Dauhatsu athletics team, but was mostly out of action due to injuries. She came back in September 2007 and won a couple of local races. She won Kenyan National Cross country championship in 2008.
At the 2008 World Cross Country Championships she finished tenth in the individual race, however, this was not enough to be a part of the Kenyan team, who won silver medals in the team competition. At the 2008 African Championships in Athletics she finished third in the 5000 metres race. She was announced as the joint winner of the inaugural World 10K Bangalore race, finishing with exactly the same time (32:02) as Elvan Abeylegesse.
Momanyi was selected to represent Kenya at the 2008 Summer Olympics in Beijing. She was the national flagbearer of the Kenyan team at the opening ceremony. She was to participate the women's 10,000 metres. However, she did not compete as she was replaced by reserve Peninah Arusei athlete under controversial circumstances. She did much to make up for her Olympic disappointment, setting a 5000 m best at the Memorial Van Damme meeting and going on to win the Great Yorkshire Run 10K race in 32:44 just two days later.
The following year she did get the opportunity to represent Kenya at a global level, running in the 10,000 m at the 2009 World Championships in Athletics. Momanyi was fourth in a close finish, running a personal best of 30:52.25. She was Kenya's second-best athlete in the race after event winner Linet Masai but managed to beat Meseret Defar, one of the pre-race favourites. Momanyi won the 2009 and 2010 editions of the Würzburger Residenzlauf 10 km race. She returned to the Bangalore 10K in May 2010, but could not repeat her past victory and instead finished in third place.
She took to the track for the 2010 Commonwealth Games in New Delhi and emerged as the 10,000 m Commonwealth champion, succeeding her compatriot Lucy Kabuu. Back on the road, she ran in the Great South Run in October and topped the podium. She saw her performance in the ten-mile race as a stepping-stone to a future in road events: "This was a good result and very important as eventually I will be moving to the marathon".
She claimed fourth place at the World's Best 10K in February 2011, but reached the podium at the Lisbon Half Marathon the following month by taking third in a personal best of 1:08:41. She was among the favourites for the Great Manchester Run in May and took third place.<ref>Wenig, Jorg (2011-05-15). Gebrselassie takes fourth Manchester 10Km victory, Clitheroe surprises. IAAF. She lives in Kisii. She has a child.
|2008||World Cross Country Championships||Edinburgh, United Kingdom||10th||Senior race|
|African Championships||Addis Ababa, Ethiopia||3rd||5000 m||15:50.19|
|World Athletics Final||Stuttgart, Germany||7th||5000 m|
|2009||World Championships||Berlin, Germany||4th||10,000 m||30:52.25|
|World Athletics Final||Thessaloniki, Greece||8th||5000 m||15:32.43|
|2010||Commonwealth Games||New Delhi, India||1st||10,000 m||32:34.11|
- The Standard, 26 March 2008: Graceful Momanyi can bring gold medal
- Krishnan, Ram. Murali (2008-05-18). Tadese the men’s 10km victor, while Abeylegesse and Momanyi share women’s spoils in Bangalore. IAAF. Retrieved on 2010-04-27.
- Makori, Elias (8 August 2008). "Olympics off to a colourful start in Beijing, China". Daily Nation. Retrieved 16 May 2018.
- Makori, Elias (2008-08-13). Arusei Picked, Vice Captain Momanyi Left Out. Daily Nation. Retrieved on 2010-04-27.
- The Standard, 22 August 2008: Coach recommended Momanyi’s exclusion
- Race History. Great Yorkshire Run. Retrieved on 2010-04-27.
- Ramsak, Bob (2009-08-15). Event Report - Women's 10,000m - Final. IAAF. Retrieved on 2009-08-15. Archived 2009-09-08.
- Wenig, Jörg (2010-04-26). Kigen and Cherop take Hamburg Marathon titles. IAAF. Retrieved on 2010-04-26.
- Krishnan, Ram. Murali (2010-05-23). Mbishei, Yimer the surprise winners in Sunfeast World 10K. IAAF. Retrieved on 2010-06-09.
- Martin, Dave (2010-10-24). Ebuya clocks the fastest ever 10 Miles in UK. IAAF. Retrieved on 2010-10-24.
- Clavelo Robinson, Javier (2011-02-28). Kitwara regains title, Ejigu notches victorious 10km debut in San Juan. IAAF. Retrieved on 2011-03-01.
- Fernandes, Antonio Manuel (2011-03-20). Tadese blazes 58:30 in Lisbon, second fastest Half Marathon ever. IAAF. Retrieved on 2011-03-20.
| Flagbearer for Kenya