Mostert, his mother (Anna Francina Mostert) and 7 siblings (Martha, Gerbrecht, Johanna, Catherina, Francois, Frederik and Willem) were forcibly removed from their razed family farm in Krugersdorp (2 May 1901) and carried off to an infamous British concentration camp and survived there until the end of the war (15 October 1902).

[2][3] After release from the concentration camp, Mostert and his family moved down to the Western Cape to live with his maternal aunt.

[4] Mostert started playing rugby at the age of thirteen and joined Somerset West RFC, where he was selected for the first team in 1918.

In 1928 New Zealand toured South Africa and Mostert was named captain of the Springboks.

In 1931–32 Mostert toured with the Springboks to Britain and Ireland, playing last international match against Scotland at Murrayfield on 16 January 1932.