Mohammed Fethaullah, 58, of no fixed address, was convicted on Thursday, 23 May following a trial at Inner London Crown Court. He was jailed for life in 2003 for the murder of his stepmother.
The jury heard that on 11 January 2017 the victim contacted police to report she had been abused in 1999 aged 14.
The victim worked after school at a grocers in Brixton that Fethaullah managed. On Friday, 18 June 1999 she had worked a shift and was waiting to collect her wages.
Fethaullah then asked her to get him some cigarettes from the front of the shop which she did. On her return she finished a drink she had left on the office desk. She immediately felt unwell and collapsed, falling over some chairs and drifting in and out of consciousness. She believe her drink was spiked.
She recalled Fethaullah putting her in his car. He drove to an unknown secluded location and she woke up to find Fethaullah on top of her her on the back seat of the vehicle. He believed she was not aware of what had happened in her state and afterwards drove her home.
A short time later the victim found out she was pregnant. She had a boyfriend and despite knowing there was a chance the baby could be the product of the abuse, the couple decided to have and raise the child.
The pair split when the boy was about two and their relationship deteriorated. In November 2016 a DNA test was carried out which proved the child was conceived as a result of the abuse.
The victim decided to report the matter to Kent Police initially, who then transferred the case to the Met.
Detectives from the Child Abuse Offences Command launched an investigation that took 28 months to bring to court as officers examined DNA and forensic evidence and tried to piece together the events of 20 years ago.
Fethaullah will be sentenced on Friday, 19 July.