Born in the United States when his father served overseas in the Royal Air Force, he moved back to the UK aged 1 year old.

He was brought up in Peterborough and then moved to Gloucestershire where he played for Sir Thomas Rich's School, Longlevens RFC and Gloucester Academy.

[2] Underhill was called up to the senior England squad by coach Eddie Jones for their 2017 summer tour of Argentina.

[4] He made his senior debut in the final test of the tour as England defeated Argentina to win the series 2–0.

[2][8] Underhill started in the 2019 Rugby World Cup Final as England were defeated by South Africa to finish runners up.