Ray Gosling, known for his BBC documentaries on social issues, said he carried out the mercy killing of the unnamed man as he lay seriously ill in hospital in "the early period of AIDS" - likely to be during the 1980s.
The 70-year-old's confession comes amid debate in Britain about whether people with terminal illnesses be allowed to commit assisted suicide.
Earlier this month, author Terry Pratchett became the latest public figure to speak out, urging the creation of special panels where seriously ill people could make the case for their right to die legally.
Pratchett himself has Alzheimer's disease.
Gosling's revelation came in a BBC program he was making about the issue. His local police in Nottinghamshire, central England, said on Tuesday they are now looking into the matter.
"In a hospital one hot afternoon, the doctor said, 'There's nothing we can do', and he was in terrible, terrible pain," he said on the show, broadcast on Monday.
"I said to the doctor, 'Leave me just for a bit' and he went away.
"I picked up the pillow and smothered him until he was dead."
Gosling told BBC radio in an emotional interview on Tuesday that he killed his ex-lover after they had agreed "a pact".
"We had this agreement that if it got like that I would end his life and that's what I did," he said.
"Sometimes you have to do brave things and, to use Nottingham language, bugger the law."
Helping someone to commit assisted suicide is against the law in Britain and can be punished with a jail term of up to 14 years.
Last year, the Director of Public Prosecutions for England and Wales, Keir Starmer, published interim guidelines on when assisted suicide cases should be prosecuted.
These said that while there were some factors which could weigh against the chances of someone being prosecuted, such as the victim asking for help, assisted suicide was still a criminal offence.
But campaigners want more clarity, although those opposed to assisted suicide say changing the law could put seriously ill people in a vulnerable position.
ETA: added vid of Ray Gosling admitting mercy killing.