As a young man, Johnston distinguished himself as a talented rugby player. In 1893, he made his international debut in a winning match against England at Lansdowne Road. Rugby was clearly in the blood, as two of his brothers also played internationally. Three years after his debut, Johnston was made a member of the Britsh Lions and travelled to South Africa to compete.
Once the Lions' tour was completed, Johnston decided to stay on in South Africa. When the Boer War broke out in 1899, he enlisted in the British military forces. His previous militarry experience in the Inniskilling Fusiliers meant that he qas quickly promoted to the rank of major.
On the 21st of October 1899, Johnston was involved in the Battle of Elandslaagte. He and another man rushed forward under heavy fire to rally the men when they were at a crucial point in the battle. He risked his life in doing so, but was successful in inspiring the men to carry on. Johnston was awarded the prestigious Victoria Cross for his bravery on that day.
A month later, at the Siege of Ladysmith, Johnston was badly wounded and had to be transported back to England for treatment.
In 1911, Johnston returned to his native Ireland and was appointed as the Commandant of the Prisoner of War camp at Oldcastle, County Meath. He worked in the prison systems for the rest of his life, eventually settling in Kilkenny where he died on the 24th of March 1950.