[3] Mountain, born Camberwell, London was educated at Sutton Valence School.

[4] Mountain finished third behind Bevil Rudd in the 880 yards event at the 1920 AAA Championships.

[5][6] The following month at the 1920 Summer Olympics, held in Antwerp, Belgium, he represented Great Britain in the 800 metres event and finished fourth, setting a British junior record.

[10] He represented Great Britain for a second time at the 1924 Summer Olympics.

After the 1924 Olympics, Mountain settled in South Africa and later became a specialist in South African geological formations and professor at Rhodes University.