[1] Alexander, born in London and originating from Streatham, she finished third behind Gladys Young in the high jump event at the 1947 WAAA Championships[2][3][4] and second behind Dorothy Tyler at the 1949 WAAA Championships.

[8] In 1950 she was trained by George Pallett[9] and took the British record on 5 August 1950 at 5 ft 6.5.

[13][14] During the 1951 WAAA Championships on 7 July 1951 at White City Stadium, Lerwill broke the world record for women's high jump, with a jump of 1.72 metres,[15] beating the previous record of 1.71 metres set by Fanny Blankers-Koen of the Netherlands on 30 May 1943 in Amsterdam.

(The record was broken on 22 May 1954 by Aleksandra Chudina of the USSR in Kiev with a jump of 1.73 metres).

The following year at the 1954 WAAA Championships, Lerwill won another WAA high jump title.