Charles tipped cabbie John Sheen, 75, with a £10 note as he pulled up at his final destination.
He was at St Bartholomew the Great, West Smithfield, to celebrate the presentation of the Royal Charter to the Worshipful Company of Hackney Carriage Drivers.
So while the Queen is in Rome to meet the Pope in the splendour of the Vatican, Charles spent his morning in the 12th century church with London cab drivers.
As he made his way in, Charles said he had a "jolly trip in the taxi".
Mr Sheen, who has been a cab driver for 45 years, drove Charles from Clarence House and at the end of the journey, he was handed a tenner - the third £10 note he has received from the prince.
He drove a taxi with Charles in the back seat in 2004 on two trips, and was tipped £10 for each journey - and he has the notes framed, he said.
When asked what Charles was like as a passenger, Mr Sheen said: "Fantastic as always."
Adding: "He's a good lad."
Talking about his conversation with Charles as they arrived at their final destination, he recalled: "He said 'John, that was an excellent ride. You did a wonderful job. This is for you'.
"He gave me a tenner."
Mr Sheen said he would get the note framed alongside the other banknotes Charles gave him.
Charles met members of the Worshipful Company of Hackney Carriage Drivers who are all professionals who earn their living through the Hackney Carriage trade and include some of London's black cab drivers.
Speaking at the service, Charles praised the cab drivers for their "sound common sense" and the quality of service they provide.
Singling them out as one of the great service providers in the country, he said: "You are an institution. You are one of its great traditions."
Charles also commented on the "extraordinary selflessness" involved in the Company's annual Children's Magical Taxi Tour.
A convoy of over 100 taxis transports chronically sick children and their families to Disneyland Paris for the weekend.
James Cannon, a nine-year-old, who was on the trip last year presented Charles with a picture of Mickey Mouse for his grandson Prince George.
James, who is currently in remission but suffered from eye cancer, said making the presentation was "very exciting".
His mother said she found the event very emotional.
Claire Cannon, from Hullbridge in Essex, said: "I was crying my eyes out. We're very proud of him.
"I didn't think we'd be standing here. He's a lovely boy."
In 2013 The Worshipful Company of Hackney Carriage Drivers were granted a Royal Charter.
The Company is the 104th Livery Company of the City of London, having received Grant of Livery in February 2004.
In 2004 Charles gave a reception for London cab drivers to pay tribute to the charity work that many of the drivers do, including their work helping underprivileged children.
Prince Charles Speach to the Worshipful Company of a Hackney Carriage Drivers.
Source: MSN news on line.
With thanks to Glen.