An ultra-trail specialist, his wins include the JFK 50 Mile in 2014, 2015 and 2016, the Lake Sonoma 50 in 2016 and 2018, the Tarawera Ultramarathon in 2017, and the Western States 100 in 2018, 2019, 2021, and 2024.

[12] Walmsley won the World Long Distance Mountain Running Championships in 2019.

[13] In 2023, Walmsley won Ultra Trail de Mont Blanc UTMB in a course record of 19:37:43 becoming the first American man to win the race.

He considered attending Arizona State, Georgetown, Iona College, Washington, California and the Air Force Academy,[15] eventually choosing the latter.

[8] In 2012, as the captain of the cross country and long-distance running teams,[8] his best times were 4:04 for the mile, 13:52 for 5000m, 29:08 for 10000m and 8:41 for the 3000m steeplechase.