Ethel was born 8 June 1879 as the second child of Herbert Warneford Thomson, surgeon, and his wife, Sophia Charlotte Bond. Her older brother Hugh was born in 1878.
She first competed at Wimbledon in 1902 when she lost in the first round to Agnes Morton. The following year, 1903, she reached the final of the All-Comers tournament in which she was defeated by Dorothea Douglass in three sets. In 1912 she became Wimbledon champion by first defeating Charlotte Cooper Sterry in the All-Comers' final and subsequently receiving a walkover in the Challenge Round.
1 This was the all-comers final as Dorothea Douglass Lambert Chambers did not defend her 1911 Wimbledon title, which resulted in the winner of the all-comers final winning the challenge round and, thus, Wimbledon in 1912 by walkover. 2 This was the all-comers final as Muriel Robb did not defend her 1902 Wimbledon title, which resulted in the winner of the all-comers final winning the challenge round and, thus, Wimbledon in 1903 by walkover.