Yomif Kejelcha made his international debut at the 2013 World Youth Championships in Donetsk, winning gold in the boys' 3000 metres with a time of 7:53.56. At the Ostrava Golden Spike elite meeting in May 2014 he placed second over 3000 metres in 7:36.28, losing only to Kenya's Caleb Ndiku; he defeated 2008 Olympic medallist Nick Willis and former World Champion Bernard Lagat. Yomif won the 5000 metres at the 2014 World Junior Championships in Eugene, Oregon, running 13:25.19; he also won gold, in the 3000 metres, at the Youth Olympic Games in Nanjing later that summer. He finished 2014 as the world youth leader at both 3000 and 5000 metres, and world junior leader at 3000 metres.
Yomif opened his 2015 season at the African Junior Championships in Addis Ababa, winning the 5000 metres in 14:31.03. He made his IAAF Diamond League debut in Doha, placing fifth in the 3000 m; he then won a non-Diamond Race 5000 m race at the Prefontaine Classic in Eugene, improving his personal best to 13:10.54 and outkicking Edwin Soi and Galen Rupp. Yomif scored his first Diamond League victory the following week at the Golden Gala in Rome, winning in a world-leading 12:58.39 and breaking 13 minutes for the first time. On September 11, 2015 Kejelcha competed in the Diamond League event in Brussels and set a new personal best (and world-leading time for 2015) of 12:53.98 in the 5,000 meter event.
He started 2016 by winning the gold in the 3000 metres at the 2016 World Indoor Championships in Portland. On 27 August 2016, Kejelcha won the 3000m at the 2016 Diamond League Lausanne meet in a new U20 world record of 7:28.19.