Tanya Perks, 31, from Telford, had parked her car outside the former Black Horse pub in Oakengates at about 10.30am on Friday, January 13, and accompanied her mother, Pauline Kemp, 67, from Ketley Bank, to the doctor's surgery.
After returning and getting inside the vehicle a large chunk of masonry fell onto the window wiper and bounced onto the bonnet.
Tanya Perks said: "It is lucky that we had just got into the car.
"If it had struck an elderly person or a baby in a buggy it could have had serious consequences.
"The falling mortar made such a thud that the woman in the car in front of me got out, thinking that I had collided with her vehicle."
She added: "The mortar had broken off and hit the roof of the vehicle where Mum was sat and it is fortunate that I do not drive a convertible.
"I have informed the environmental health department with Telford & Wrekin Council."
Her mother said: "We cannot work out exactly where it fell from but it looks like guttering on the Black Horse building in Oakengates which closed in 2007 after a fire.
"We knocked at the door because we could see a light inside but there was no reply.
"Staff at Giles Opticians nearby heard the thud.
"I had been to the doctor's surgery for a blood test and had just got back into the car when there was a loud thud and a large piece of concrete had fallen on the roof and landed on the windscreen wipers.
"It bounced off and dented the bonnet and also scratched the paintwork."
She added: "I understand that this is not the first time this has happened.
"I think that something must be done because someone might be injured or even killed.
"We wanted to trace the owner of the building but all we could discover was that they are believed to live in Manchester.
"We have reported what happened to the environmental department of Telford & Wrekin Council."
A spokesman for Telford & Wrekin Council said: “Our Building Control Team have visited the Black Horse pub site in Oakengates to check the reported danger.
“There appears to be no obvious or immediate danger.
"The Building Control Team have contacted the property services company, who manage the building on behalf of the private owner, who have confirmed that they will be visiting the building later this week to assess fully and make any repairs necessary.”