Agnes Tirop first gained national attention in 2012, when she finished second to world junior champion Faith Chepngetich Kipyegon in the Kenyan Cross Country Championships. This resulted in her first national selection and international medal at the 2012 African Cross Country Championships, where she finished second to Kipyegon and won the junior silver medal. She was Kenya’s most prominent entrant in the 5000m at the 2012 World Junior Championships in Athletics, where she finished third in a personal best of 15:36.74 minutes.

Tirop finished second to Kipyegon again in the 2013 Kenyan Cross Country Championships, and their collaboration led to a Kenyan 1–2 and team title at the 2013 IAAF World Cross Country Championships.

Kipyegon successfully defended her title, while Tirop finished a close second to win her first medal of the competition. That year, she improved on the track, setting personal bests of 8:39.13 minutes for the 3000 metres and 14:50.36 minutes for the 5000 metres, as well as on the roads, where she ran a half marathon in 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 (Kipyegon won both senior races).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 debuted in the senior ranks in 2015 and made an immediate impact, winning the Eldoret Discovery Cross Country in Kenya. She finished second to Kipyegon in the Kenyan senior national championship race and was selected for the senior national team. Tirop took to the front and gradually moved away from the field to win the senior gold medal by five seconds at the 2015 IAAF World Cross Country Championships, her senior international debut as the fourth youngest athlete in the field. This made the 19-year-old the second-youngest winner of that title in championships history, trailing only Zola Budd’s win in 1985, and gave Kenya its 300th medal at the competition.

In 2017, she competed in the World Championships in London, where she won bronze in the 10,000 metres with a time of 31:03.50, her personal best in the distance. In 2018, she set a course record in the World 10K Bangalore race. At the 2019 World Athletics Championships, she took bronze in the same event.

Agnes Jebet Tirop finished fourth in the 5000m event at the postponed 2020 Summer Olympics. She broke the world record in a 10 kilometre women-only event in September 2021. In an event in Herzogenaurach, Germany, she set a time of 30:01. In October 2021, she finished second in the Giants Geneva race, trailing only Kalkidan Gezahegne, with a time of 30:20.