Norman Pender, 65, of Hawick in the Borders, was convicted of three charges of lewd and libidinous behaviour and one of sexual assault. A charge of raping one of the girls was found not proven.
The offences were committed at various address in Hawick in the 1980s and 1990s.
Pender was convicted after a trial during which he pleaded not guilty, forcing his victims to give evidence against him. But yesterday it emerged he had admitted sexually abusing the girls. He went to trial because he denied the rape allegation, but admitted to social workers he had had sex with a girl when she was over 16, and claimed it was consensual.
Jailing Pender at the High Court in Paisley, judge Lord Stewart told him: "This is a terrible fall from grace.
"These were innocent victims and you were in a position of trust. There seems to be an unfounded belief in some quarters that you are completely innocent and these girls made up these allegations."
One of his victims, now aged 35, said in evidence she repeatedly told Pender "no" and tried to fight him off, but he was too strong. She added: "I felt dirty, sick, lonely and ashamed. I knew it was wrong. He told me not to say anything. I was scared to tell anyone.
"He said I was a child and nobody would believe me. I tried to stop him doing what he was doing, but he just did it anyway.
"I was terrified of annoying him because when he kicked off I was terrified of him."
When asked by advocate depute Alison Di Rollo, prosecuting, how often the sexual abuse took place, she replied: "Far too many times to count."
Pender abused his second victim, now 26, from the age of 10 for a year. He sexually abused her between 10 and 15 times, the court heard.
Pender served as a LibDem councillor on Scottish Borders Council between 1998 and 2003. He was also chairman of the Hawick Lady Riders Association and campaigned for women to ride in the town's Common Riding Festival.
The prop forward, who was capped for his country in the 1970s and played for Hawick RFC, preyed on the young girls between April 1986 and October 1997.
He abused his first victim from when she was eight until 19 and began abusing his second victim in January 1996 when she was nine.
Pender is a first offender whose abuse came to light when one of his victims went to the police.
Ms Di Rollo, who is head of the National Sexual Crimes Unit, told the jury: "This trial has shone a light on dark and disturbing secrets.
"What Mr Pender did was desperately and wholly wrong. By silencing his two victims in a climate of fear, he succeeded in the callous and calculated abuse of two girls."
Pender did not give evidence in court but claimed both his victims had made up lies about him.
Advocate Iain Paterson, defending, said his client had raised tens of thousands of pounds for worthy causes. Mr Paterson added: "The reason he went to trial was the rape allegation against him."
Pender was also placed on the sex offenders' register.