Catherine Williamson, from Leamington Spa, visited Hartlepool marina to meet her 25-year-old son Joseph Thompson as he completed a 30-mile run down the coast.
He was supporting his friend Paul Minter, who is running 5,000 miles around Britain between March and October to raise money for Head Up, a mental health charity which supports forces servicemen with PTSD.
Catherine parked on Navigation Point as she cheered them over the line and for a celebratory drink.
But she did not realise it was pay and display and four days later received a £100 penalty charge, reduced to £60 if paid in two weeks.
Catherine tried to appeal it but it was upheld by the car parking management firm.
She described it as a “sucker punch” after an otherwise positive time in Hartlepool.
Catherine said: “The restaurants were lovely, the people were warm and welcoming.
"Having had such a positive experience to get walloped with that fine, it was like a sucker punch.
"I cannot imagine it is good for the marina.”
She added: "I was completely oblivious to the fact I had got into a pay and display area.
"I explained I wasn’t familiar with the area. It didn’t look like an area where you were expected to pay.
"It was a bit gutting with what my son had achieved and it’s quite incredible what Paul is doing.”
Catherine offered to donate the cost of the parking charge to the charity if refunded.
But Northwest Parking Management, which manages parking on the marina, said tariffs are clearly displayed throughout the site including 48 Terms and Conditions signs, 15 of which are located along the shop fronts where Mrs Williamson parked.
The company added: “All signs have been audited and approved by the British Parking Association (BPA) and this site has also been fully audited and approved by a BPA auditor in December 2021.
“We do not comment on individual cases, however if an appeal has been submitted to us and subsequently declined the motorist has the opportunity to then submit a further appeal to POPLA, the independent adjudication service.”