Robert Gillings, 63, who was granted bail pending sentencing, was a guest at the television personality's holiday home in the New Forest, Hampshire, when he assaulted the girl as she played in the pool, Guildford Crown Court was told. Ms Rantzen was not at the house at the time.
The victim told the jury that the defendant had also assaulted her before the trip to the New Forest. 'He had said if I told mummy he would kill her,' she said.
Gillings denied the allegations and claimed that Ms Rantzen's television work on behalf of abused children must have put ideas into the girl's head.
He said of the swimming pool incident: 'We were playing tag at the time, swimming backwards and forwards. My fingers caught hold of her bikini just at the bottom or the side somewhere. It was just a bit of fun - it was such an insignificant thing.'
However, the jury of seven women and five men returned a majority verdict after six hours, convicting the defendant of two charges of indecent assault, one of which took place at Ms Rantzen's house. But they cleared him of two other charges of indecent assault, one in which the girl claimed he had molested her in Ms Rantzen's changing room.
After the case, Ms Rantzen, best known for the long-running television series That's Life, said her holiday home had been tainted by the incident.
She said: 'My daughter suggested we should get an exorcist. It is a happy place for us, or has been. I feel that this place which my family and I have loved - and these children have loved - has indeed been blighted.'
Ms Rantzen said it was 'appalling' to suggest that she had put ideas into the girl's mind.
The court was told that the victim's mother used to work for Ms Rantzen and had been invited to use the country house for a break last summer. The family, who were good friends of Gillings and his wife, invited the retired couple down for the day. The assault took place as a barbeque was being prepared.
Gillings, of Addlestone, Surrey, allegedly told police: 'To my mind, she (Esther) is a little bit into this sort of thing. It was probably her that has put her mother up to this.'
He added: 'She has had that notion put into her in some way, I am sure.'
Susan Tapping, for the prosecution, said that the maximum sentence Gillings could receive for indecent assault was 10 years in jail. He will be sentenced on 29 July.
Discussing the condition of the girl, she said: 'I believe some counselling is intended. I know she had an early birthday party this weekend before school breaks up and I believe it went well.'
After the verdict, the girl's father said: 'We are satisfied with the verdict, we just want to rebuild our lives with our children.'Reuse content