John Rarity, 67, was aged just 17-years-old when he repeatedly had intercourse with a 12-year-old female in Broxburn, West Lothian, in March 1966.
The High Court in Edinburgh heard on Tuesday how Rarity had started sexually assaulting his victim, three years earlier.
He continued to have illegal sexual relations with the woman, who cannot be named for legal reasons, until March 1967.
However, the court heard the woman didn’t contact the police about the allegations until many years after the abuse took place.
Police then built their case against Rarity and collected enough evidence to take him to court.
Rarity, of Dechmont, West Lothian, pleaded guilty to sexual assault and underage sex charges at an earlier hearing. Sentence had been deferred for the court to obtain reports.
On Tuesday, defence solicitor advocate Raymond McMenamin told the court that his client regretted his actions.
He added: “It was his precise instructions that he did not want this case to go to trial.”
Judge Lord Bannatyne told Rarity that he had no other option but to send him to prison.
He added: “These are serious offences. I would be failing in my public duty if I did not impose a term of custody.”