Slawomir Zakrzewski has been convicted of the manslaughter of Goode Foods owner Gloria Burgos at the end of a two week trial at the Old Bailey.
He was cleared of murder and robbery which he denied.
The body of 54 year old Colombian-born Gloria Burgos was found in her Goode Foods sandwich shop in Bermondsey Street last September. She had suffered 40 wounds.
Zakrzewski, who had been working at the cafe for a year until being sacked, returned to obtain a reference.
In his defence, Zakrzewski said he pushed her during a row resulting in her falling. He kicked her knowing she was injured but told the court that he did not know she would die from her injuries.
Her body was found five hours later.
Prosecuting counsel Jonathan Rees told the court that Mrs Burgos suffered a "sustained and savage" attack and received 40 separate external injuries, "including a severe brain injury and a collapsed lung". Zakezewski targeted her head, leaving "the imprint of the sole of his shoe" on her cheek.
Sentencing of 23 year old Zakezewski will be on Friday 14 August.
Earlier this year a maple tree was planted in memory of Gloria Burgos in the park opposite her cafe.