During the Second Boer War, Johnston was awarded the Victoria Cross while serving with the Imperial Light Horse.

Johnston was born at Laputa, near the townland of Clyhore (or Cloughore), between Ballyshannon and Belleek in the south of County Donegal in Ulster.

[7] When the British Lions tour ended, Johnston, together with Thomas Crean, decided to stay on in South Africa.

In 1899, at the start of the Second Boer War, again with Crean, he enlisted in the Imperial Light Horse and Johnston subsequently reached the rank of major.

In October 1899, according to the Irish Times, the Imperial Light Horse particularly distinguished themselves in the Battle of Elandslaagte and it was during this engagement that Johnston, while still a captain, won his VC.

The joint citation for him and Captain Charles Mullins reads: On the 21st October, 1899, at Elandslaagte, at a most critical moment, the advance being momentarily checked by a very severe fire at point blank range, these two Officers very gallantly rushed forward under this heavy fire and rallied the men, thus enabling the flanking movement, which decided the day, to be carried out.

He travelled to London in early 1901, and both he and Mullins received the VC from King Edward during an investiture at Marlborough House 25 July 1901.

In 2019 his story along with other Wanderers Victoria Cross recipients Thomas Crean and Frederick Harvey was told in a documentary entitled "Mark Our Place" directed and Produced by Ashley Morrison.