Mark Henry, 26, of Kilburn, north-west London, was cleared of attempting to murder Doris Thompson.
The Old Bailey jury also convicted him of affray relating to an incident 24 hours before the assault on Mrs Thompson, when he approached another woman in a supermarket car park four miles away. He raised the baseball bat as if to hit her but backed off when other shoppers appeared.
Henry, who has been in a hospital secure unit since his arrest, was remanded for reports before sentencing.
The Common Serjeant of London, Judge Neil Denison, said because of Henry's history of violence he was considering an indefinite sentence.
The court had been told Mrs Thompson, who lives in Belgravia, was lucky to survive the head injuries she received when Henry attacked her in a Sainsbury car park in Wandsworth, south-west London. Mrs Thompson said she still suffered 'dramatically' from the attack and had to wear a wig, but she had no memory of the actual assault or of her treatment in hospital.
Henry, who has been diagnosed as suffering from schizophrenia, denied the charges but did not give evidence. After the verdicts the jury was told that he had convictions for violence and threatening behaviour dating back to 1987.Reuse content