Tirop first rose to prominence at national level in 2012, when she was runner-up to world junior champion Faith Chepngetich Kipyegon at the Kenyan Cross Country Championships. This led to her first national selection and international medal at the 2012 African Cross Country Championships; she was again runner-up to Kipyegon and took the junior silver medal. She was Kenya's most prominent entrant for the 5000 metres at the 2012 World Junior Championships in Athletics and finished with a bronze medal in a personal best of 15:36.74 minutes behind Ethiopian opposition. Tirop was again second to Kipyegon at the 2013 Kenyan Cross Country Championships and teamwork between the pair led to a Kenyan 1–2 and team title at the 2013 IAAF World Cross Country Championships – Kipyegon defended her title while Tirop was a narrow second to claim her first medal at the competition. She made progress on the track that year, setting personal bests of 8:39.13 minutes for the 3000 metres and 14:50.36 minutes for the 5000 metres, and also on the roads with a half marathon best of 71:57 minutes. In the 2014 season she finally emerged from Kipyegon's shadow. Tirop won the Kenyan cross country junior title and then dominated the junior race at the 2014 African Cross Country Championships, leading the Kenyans to victory by a 14-second margin. She was unable to achieve such a margin over African runner-up Alemitu Heroye at the 2014 World Junior Championships in Athletics and was again third in the 5000 m, while the Ethiopians extended Kenya's historic lack of a gold medal in that event. Tirop entered the senior ranks in the 2015 season and immediately performed well, winning the Eldoret Discovery Cross Country in Kenya. She was second to Kipyegon at the Kenyan senior national championship race and earned a senior national selection – a performance which filled her with confidence: "I did not even believe I could make the team. I will not fear running against seniors." For the 2015 IAAF World Cross Country Championships, after Kipyegon withdrew the reigning world championEmily Chebet was seen as Kenya's leading athlete, and Tirop as a key team member. Despite this being her senior international debut and the fourth youngest athlete in the field, Tirop took to the front and gradually moved away from the field to win the senior gold medal some five seconds ahead of Ethiopia's Senbere Teferi. This made the 19-year-old the second-youngest winner of that title in championships history, after Zola Budd's win in 1985, and also brought her Kenya's 300th medal at the competition. With Ethiopia rounding out the top four and defending champion Chebet in sixth, Kenya were beaten into second in the team race. In 2017 she participated in the World Championships held in London, winning the bronze medal in the 10,000 metres event, with a time of 31:03.50, her personal best in the distance.
