Sum was born in the village of Kesses in Uasin Gishu County in the former Rift Valley Province and competed in the heptathlon and handball at school. She gave birth to a daughter Diana Jeruto in 2008 and only began a full-time athletics career in 2009 after being spotted competing in a heptathlon event by former 800 metres world champion Janeth Jepkosgei. At her invitation, Sum moved to Eldoret to train with Jepkosgei's group.[2][3]
Sum won a silver medal – her first medal in international competition – in the 800 m at the 2012 African Championships in Athletics in Porto-Novo, Benin. She ran a personal best of 1:59.13 in the final, finishing two hundredths of a second behind Burundi's Francine Niyonsaba.[4] Sum then competed in the 1500 m at the London Olympics, but finished a disappointing 10th in her heat and failed to qualify for the semi-finals.[3]
Sum progressed to the highest ranks of international competition in 2013, reaching her first global final and defeating Olympic champion and home favourite Mariya Savinova to win a surprise 800 m gold in a personal best time of 1:57.38 at the World Championships in Moscow.[5] The Kenyan defeated Savinova again at the Weltklasse Zürich meeting to add the 2013 Diamond League crown in the 800 m to her world title.[6] She won the 800 m at the 2014 Commonwealth Games.[7]
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