Mr Johnstone joined the Royal Irish Rangers (Reserve) in 1988.
He was commissioned from Sandhurst in 1991 and attained the rank of major before leaving the Army in 2014.
In 2004 he was deployed to Iraq where he served as second in command of his company in Basra.
He said it was an honour and a privilege to be appointed veterans commissioner.
“This is a very important role and one that can make a positive difference to the lives of veterans and their families,” he said.
“I aim to hit the ground running in the new year, by getting out and engaging with the wider veteran community and key partners, alongside the experienced NIVCO team.”