A jury found Stuart Anzac Stone guilty in March this year.

On Thursday in the ACT Supreme Court, Justice John Burns sentenced Stone to three years and nine months' imprisonment with nine months to be served full-time.

Stone indecently assaulted the boy on two occasions, the court heard.

One night in 1975, he was at the Farrer house of a friend he had met through church. The friend's boy was seven at the time, and Stone entered his bedroom after he had gone to bed and molested him.

He told the boy everything was okay, and that it was a secret.

On the second occasion in 1977, the boy's family had come to Stone's house for dinner. Stone again indecently assaulted the boy.

Stone, now 76 and living in Victoria, is an ill man, the court heard.

Expert evidence showed he was in remission from advanced stage four lymphoma after several rounds of chemotherapy in 2010.

He had recurring lung infections, and received monthly antibody injections to stave off illness. He had developed melanoma, and had chronic lower back pain, asthma and arthritis.

In a victim impact statement tendered to the court at an earlier date, the victim said the assault had had a profound impact on him.

He found it difficult to trust people, or let anyone near his children.

The court heard Stone was "mortified" by what he did, and felt a profound shame in admitting the assault was sexual in nature.

His wife of 47 years had stood by him during the trial.

Justice Burns said he was satisfied the man was a low-risk of reoffending, and he had "effectively rehabilitated himself".

But the crime was serious, because there was a gross breach of trust.

He had breached the trust of the boy and his family, and the only reason he had access to the boy was because of that trust.

The boy was entitled to feel safe in his own home, Justice Burns said.

Stone was sentenced to three years and nine months imprisonment. He will serve nine months before being released on a good behaviour order.

He is due for release in May 2017.