Sabastian Sawe was raised by his grandmother Esther in a farming family in Ndonyongaria village, Nandi County. He ran as a schoolboy but had little early success, initially focusing on middle distances before shifting to road races around 2017. The turning point came at the 2022 Sevilla Half Marathon, where he had been hired as a pacemaker. Despite never having raced beyond 5000m in competition, Sawe kept running past his pacing duties, won the race in 59:02, and lowered the course record by ninety seconds. From there, he joined Italian coach Claudio Berardelli's 2 Running Club in Kapsabet, a high-altitude training group that also houses Olympic 800m champion Emmanuel Wanyonyi. Under Berardelli, Sawe developed rapidly. In October 2023, he won the World Road Running Championships half marathon in Riga in 59:10, a championship record. His marathon debut at Valencia in December 2024 produced a world-leading 2:02:05. In 2025, he backed it up with wins at London (2:02:27) and Berlin (2:02:16), finishing the year as the world's top-ranked marathoner and World Athletics Out-of-Stadium Athlete of the Year. He has won all three of his career marathons, each in under 2:03.