The model wife of rock star Sir Rod Stewart - who qualified as a Special Constable volunteer police officer in 2021 - worked with the City Of London police marshalling the crowds outside RAF Northolt, where the plane carrying the late Queen landed after leaving Scotland.
Queen Elizabeth II died at the age of 96 on Thursday 8 September at her beloved Scottish residence Balmoral Castle.
The coffin had been Lying-In-State at Holyrood House in Edinburgh before it was flown to London, accompanied by Queen Elizabeth's daughter Princess Anne, to rest at Buckingham Palace overnight before it is moved to Westminster Abbey.
Lancaster, 51, was seen helping a member of the public in a wheelchair, as well as assisting another with the fold-up chair they had brought with them while they lined the streets to pay their respects to the late Queen.
The Loose Women panellist told Good Morning Britain: "I have extreme pride to serve on the streets of London on Friday and I will be again on Wednesday when the Queen's cortege and on Monday, which is a historic moment I'll be very proud to be at."
Lancaster also recalled dancing with King Charles III at his 60th birthday party, to husband Rod Stewart's hit 'Da Ya Think I'm Sexy?'
She said: "It was his 60th birthday and he had asked Rod to perform and he asked if I would be the first on the dance floor because you can imagine people being nervous and not knowing the protocol, who should get up first.
"So he asked in the drink reception if I would do the first dance, so it would get everyone up.
"While Rod was performing I was on the dance floor with Prince Charles, Prince Andrew, William and Harry. It was an incredible family event."
Lancaster qualified as a police officer in 2021 and regularly patrols the streets of London as part of her 200-hours-a-year volunteer role.
She revealed in March that she had made her first arrest, after stopping a driver who tested positive for being under the influence of cannabis.
Lancaster confessed: "I did get a bit of a stage struck moment where I felt all the words of the caution had left me but luckily they came into action and all my training proved important there."
The model also revealed she had saved someone from taking their own life while patrolling a bridge.
Watch: Penny Lancaster breaks down talking about the menopause