At the 2013 IAAF World Cross Country Championships – Junior men's race he finished 41st.

[4][5] Birtwhistle finished second at the Hamburg[6] and Edmonton[7] events of the 2017 ITU World Triathlon Series.

[9] In the 2018 ITU World Triathlon Series he took silver in Yokohama[10] and bronze in Edmonton[11] and Montréal.

[12] He completed the year with 7th at the 2018 ITU World Triathlon Grand Final, also on the Gold Coast.

[13] 2019 saw Birtwhistle take his first gold at an ITU Triathlon World Series race, winning in Leeds[14] (standard distance) and subsequently taking first place in Hamburg[15] later that season (sprint distance).