Nyambura was selected for the Kenyan distance medley relay for the 2015 IAAF World Relays – her first senior national call-up. She anchored the team to the silver medals behind a world-record breaking American team. As part of the Kenyan team, she helped break the African record and Commonwealth record with their time of 10:43.35 minutes. She had her first major individual victory at the Doha leg of the 2015 IAAF Diamond League. Charged with pacing duties, she led up to 2000 m, then continued the race to score a victory in a personal best of 9:21.51 minutes.