Tracy Mitchell, who collected Kate Middleton's parents from the Lindo Wing on Tuesday afternoon, described them as "very down to earth people".
As proud first-time grandmother Carole spoke to the press about their "absolutely beautiful" grandson, Tracy had no idea who was about to get in the back of her cab.
Tracy's bright blue cab can be seen in the background
The 49-year-old, who has been a taxi driver since January, drove the thrilled pair back to the Duke and Duchess of Cambridge's London residence at Kensington Palace.
"They chatted amongst themselves and she spent much of the journey on the phone," Tracy told the Telegraph. "When we got to the other end I congratulated them on the arrival.
"It was the most important passenger I’ve ever had. Our future king's grand-parents — you don't get much more important than that other than real royalty."
Tracy received a call at 4pm and was told to go to St Mary's Hospital, Paddington where William and Kate had welcomed their baby son into the world on Monday.
However, she did not realise that she would be picking up the grandparents of the future King on the momentous day he was introduced to the world.
"I had my hair up and sunglasses on my head and I thought 'Oh my god'," she said.
"I said to the press officer: 'I wish I’d have known I was going to get this call today — I’d have done my hair'."
"The next thing I get another call from a press officer asking me what car I was in, the colour of the cab and that sort of thing.
"When I got there it was like Secret Squirrel — you're going to do this, you're going to do that."
"It wasn't until I got there and I was told that it was Kate’s mum and dad I was picking up, well, you really could have knocked me down with a feather to be honest with you. I was really quite taken aback by it."
"I took them through Hyde Park where there are speed humps," she told the Evening Standard. "I was conscious of not bumping them about and of slowing down. I was on my best behaviour."