Atkinson stood by and watched Robert Hamill being attacked.
Robert Hamill, a 25-year-old Catholic, died after he was beaten by loyalists in Portadown, County Armagh.
Robert Cecil Atkinson, 71, from Brownstown Road in Portadown, appeared before Craigavon Crown Court on Friday. The former reserve constable admitted giving false information to police who were investigating a phone call made from his home on 27 April, the day of the attack.
Mr Hamill died of his injuries on 8 May 1997.
On Friday, the judge said Atkinson had been "a disgrace to [his] uniform".
After the sentencing, Mr Hamill's family called for a date to be set for the release of the findings of
a public inquiry into his death. His sister, Diane Hamill, said there has been "a lot of wrong doing and cover up", adding that "hopefully the inquiry will shed more light on that".
She said: “This could have been over decades ago if people had been honest about what they did."
Mr Hamill and a friend were attacked and beaten by loyalists as they walked along Portadown’s Market Street after a night out. Mr Hamill, a father-of-three, suffered serious head injuries and died 11 days after the attack.
Prosecution barrister Toby Hedworth KC previously told the court that when the attack occurred at about 01:45 BST "an RUC land rover was parked nearby" and "one of the crew was Robert Atkinson".
Atkinson was a reserve constable at the time.
He had pleaded guilty in April 2024 to conspiring with Andrea Louise McKee and James Michael Robert McKee, who have previously been convicted, as well as others to do an act which had a tendency to pervert the course of justice.
This related to giving false information in relation to the identity of the person making the phone call.
https://www.bbc.com/news/articles/c8995q8x2njo