[1] Borthwick married a descendant of the 1820 Settlers, Elizabeth Edith Walton in Grahamstown on 17 March 1893.
Borthwick began his career in South Africa on 27 March 1889 as the first assistant to Duncan Hutcheon,[4] Colonial Veterinary Surgeon to the Cape of Good Hope.
[5] Jotello Festiri Soga the first black South African vet worked with him as Hutcheon's second assistant.
[6] In 1892, he joined Alexander Edington in his laboratory (previously buildings in the Royal Engineers yard[a]) in Grahamstown.
Borthwick served as Edington's assistant in the study of animal diseases until 1893 when he was succeeded by Thomas Bowhill.