He was raised in Baragoi, Samburu County, by his mother and grandmother after losing his father to cattle rustling when he was young. He has stated that while attending Aiyam Day Secondary School, a 24 km trek to the learning institution and back home would eventually make him an athlete because walking to school early in the morning and back in the evening was the most dangerous time to do the journey as they crossed bandits' battle grounds along the narrow paths to the school and even though he would set off at 5.30am for an 8am start at school, he was sometimes forced to take an even longer route. He is now training in Iten, Kenya, in the Rift Valley.
He placed second in the 2019 Kenya national World Championships trials. He was unable to compete, however, because he did not meet some of the Athletes Integrity Unit (AIU) doping requirements. Simiyu took the in-competition test multiple times, but did not complete the required three out-of-competition tests, which include both urine and blood and must include at least one Athlete Biological Passport (ABP) test and one Erythropoietin (Epo) test. Simiyu blamed Athletics Kenya for the disaster, claiming that the federation failed to inform him of the rules before he signed up for the trials.
He was so disappointed that he considered quitting the track and going full-time into road racing, and he won the Safaricom Kisii 10 km road race in 29:16.71, as well as the 10 km race at the Nairobi Marathon in an event record time of 28:23.
On January 3rd, he won the elite-only San Silvestre Vallecana 10 km in Spain, kicking off the 2021 season with a victory.
He was confirmed on the Kenyan team for the 2020 Summer Games in the 5000m after finishing in the top two at the Kenyan Olympic trials with a personal best 13:05.05. behind Nicholas Kimeli.
He finished fourth over 3000m at the 2022 World Athletics Indoor Championships in Belgrade in March 2022. He went on to win the Kenyan 5000m title and silver at the 2022 African Athletics Championships. He has qualified for the 5000m final at the World Athletics Championships in Eugene, Oregon in 2023. He took silver in the 10,000m at the 2022 Commonwealth Games in Birmingham.
He finished second in the 10,000m race at the 2023 World Athletics Championships in Budapest, earning a silver medal.