Mostert debuted for the Springboks on 13 August 1921, against New Zealand in Dunedin. He played as tighthead prop in the first and second test matches and then moved to hooker for the third test. The second test series in which Mostert played was the 1924 test series against the touring team from the British Isles. During the second test in the series, he scored his first and only test try.