Wanjiru made her international debut at the 2015 African Games and won a silver medal in the 5000 m, forming a Kenyan sweep of the medals alongside Margaret Chelimo and Alice Aprot.[5] The year after she was beaten into second place at the Japan Corporate Championships by another Kenyan, Ann Karindi Mwangi. She repeated as champion at the Oda Memorial and Sanyo Women's 10K. In 2017 and 2018 she won both the Japan Corporate title and the Oda Memorial 5000 m.[4]
Two years later, Wanjiru produced the second-fastest ever women's marathon debut at the Berlin Marathon with a time of 2:18:00. She went under the previous course record and finished second.[8][9]
In March 2023, Wanjiru won the Tokyo Marathon with a 2:16:28 clocking, taking more than a minute and a half off her personal best to move up to sixth on the event's world all-time list. This was only the second marathon race of her career.[1][10]
^"Women's Half Marathon"(PDF). 2020 World Athletics Half Marathon Championships. Archived(PDF) from the original on 17 October 2020. Retrieved 17 October 2020.