David Jonathan Holden, 53, had denied the manslaughter of Aidan McAnespie in February 1988 at his trial at Belfast Crown Court.
McAnespie, 23, was shot in the back at a checkpoint in Aughnacloy, Co Tyrone, during the Troubles.
A bullet fired from some 300 metres away ricocheted off the road surface, striking McAnespie in the back, moments after he walked through a border security checkpoint on his way to a local Gaelic Athletic Association club.
In 2009, the UK government said his death was a matter of “deep regret”.
Mr Holden, a British Army soldier who was aged 18 at the time, insisted it was an accident and claimed his hands were wet and that his finger slipped on the trigger of his machine gun.
He was charged with manslaughter in 1988 but the charge was later dropped. But trial judge Mr Justice O’Hara said he was satisfied beyond reasonable doubt that Mr Holden was guilty of manslaughter by gross negligence.
He said that Mr Holden should have appreciated the consequences of his actions the moment he pulled the trigger.
The case was heard in a Diplock format without a jury sitting. Supporters for Holden gathered outside the court each day the trial sat.
The trial was held amid continuing controversy over government plans to deal with Northern Ireland’s problematic past.
The Northern Ireland Troubles (Legacy and Reconciliation) Bill proposals provide an effective amnesty for those suspected of killings during the conflict if they agree to cooperate with a new body known as the Independent Commission for Reconciliation and Information Recovery (ICRIR).
The bill would also prohibit future civil cases and inquests related to crimes committed during the Troubles.
Speaking outside court, Paul Young, national spokesman for the Northern Ireland Veterans Movement, said veterans will be “deeply disappointed by this verdict”, adding that he understood Holden would appeal.
“I understand that the family are going to feel completely different to us veterans, and they will have their time to say what they say.
“But for us, the witch hunt continues, that’s why we support this legacy bill that is going through parliament right now which will stop any further prosecutions of veterans that have been previously investigated.
“The terrorists have effectively got an amnesty ... with letters of comfort, royal pardons.
“This witch hunt continues but that’s why we’re hopeful that the bill will get passed into law next year.”