Father Anthony McSweeney, 68, of Old Brighton Road, Pease Pottage, was also found guilty of three counts of making indecent images of children at Southwark Crown Court last month.
He faces sentencing today (Friday March 27).
McSweeney was cleared of three indecent assaults on two other boys at the London care home and taking a pornographic photo of one of the boys.
Speaking at the end of the trial last month, Judge Alistair McCreath said: “You are not to come to court thinking to yourself that the judge gave me bail and therefore I’m not to go to prison, because that would be a false hope.”
The court heard that McSweeney was close friends with John Stingemore, the manager of a care home in west London between 1979 and 1981.
Stingemore was found dead in January aged 72.
Stingemore provided McSweeney access to children at the care home, and both men molested boys, the court heard.
He confessed to watching a child porn film with Stingemore, but claimed he turned it off.
Responding to the verdict, Gillian Norton, chief executive of Richmond Council, said: “The council is sorry that a child in its care was indecently assaulted.
“The assault happened 35 years ago and clearly the service leadership and management laid bare in court were totally unacceptable.
“The situation today is completely different. Most looked-after children are in foster homes. Only those children needing very specialist services are placed in children’s homes.”
McSweeney was arrested in 2013 after police began investigating abuse following revelations about Jimmy Savile.
The priest was once enlisted by TV cook Delia Smith to hold a special football service at Norwich City, and he officiated at the 1990 wedding of boxing star Frank Bruno and his now ex-wife Laura.
Acting detective chief inspector Keith Braithwaite, from the sexual offences, exploitation and child abuse command, said: “McSweeney was an abuser who used his role in a position of trust.
“I would like to pay tribute to the courage of the victim in speaking out against McSweeney. It was that testimonial that secured the charges and enabled this case to be brought to court.
“I would also like to thank those witnesses who provided key evidence that ensured the court could hear what sort of man McSweeney really is.”