His international debut came at the 2010 IAAF World Cross Country Championships, where he came 18th and helped Uganda to the junior bronze team medals. His junior career took off the following year as he was the junior silver medallist behind Geoffrey Kipsang at the 2011 IAAF World Cross Country Championships, then won bronze medals in the 5000 metres and 10,000 metres at the 2011 African Junior Athletics Championships. Ayeko stepped into the senior category in 2012 and began to focus on track running, running a 5000 m best of 13:23.25 minutes for second at the Memorial Primo Nebiolo and a 10,000 m best of 27:43.22 minutes in Birmingham.
After his Olympic debut, he began 2013 on grass and won the Antrim Cross Country before taking second at the Cross de San Sebastián. He placed fourth at the Cinque Mulini a month later. He was runner-up to Peter Kibet at that year's Ugandan 10,000 m championship race and went on to place eleventh at the 2013 World Championships in Athletics with a personal best run of 27:40.96 minutes. He made his half marathon debut at the Great Birmingham Run in October and won the race in a time of 1:02:32 hours. A week later he came third at the Great South Run 10-miler.
- Thomas Ayeko. London 2012. Retrieved on 2013-02-18.
- "Athletes – Ayeko Thomas Biography". iaaf.org. Archived from the original on 8 September 2012.
- Word Cross Country Championships 2010. World Junior Athletics History. Retrieved on 2013-02-18.
- Word Cross Country Championships 2011. World Junior Athletics History. Retrieved on 2013-02-18.
- African Junior Championships 2011. World Junior Athletics History. Retrieved on 2013-02-18.
- Thomas Ayeko. IAAF. Retrieved on 2013-02-18.
- Duffy, Cóilín (2013-01-12). Ayeko and Britton take the honours in Antrim. IAAF. Retrieved on 2013-02-18.
- Sampaolo, Diego (2013-02-03). Ethiopia's Edris and Godfay take the Cinque Mulini honours. IAAF. Retrieved on 2013-10-21.
- Hardman, Dean (2013-10-20). Steel breaks course record, Ayeko makes promising debut in Birmingham. IAAF. Retrieved on 2013-10-21.
- Kiplagat and Bett battle strong winds at Great South Run. IAAF (2013-10-27). Retrieved on 2013-10-28.