The King met Richard Brock in Lancaster, where he was greeted by cheering crowds for his first official visit as monarch, reports the BBC.
He remembered the veteran from a visit to Normandy last year, and told him "you are fantastic" while at a reception at Lancaster Castle.
Mr Brock's son Tony Brock, who attended with him, said the King "mentioned a nip of whisky and said to keep taking it."
The King, who is also the Duke of Lancaster, was greeted at the railway station from the royal train by the Lord-Lieutenant of Lancashire, Amanda Parker.
He was also handed the keys to the castle in front of the John O'Gaunt gateway as part of an ancient ceremonial tradition.
The monarch has taken part in the Ceremony of the Keys when visiting the castle since 1851, when keys were first presented to Queen Victoria, with Queen Elizabeth II taking part in 2015.
Pupils from Willow Lane Primary School were chosen to go to the ceremony and greet his majesty.
Schoolchildren from local secondary schools including Ripley St Thomas and Lancaster Girls' Grammar School lined the route outside the castle accompanied by a performance from a military band.
After the ceremony, the King met with local business representatives who were showcasing their work.
He expressed his love of cheese, telling Gillian Hale, from Butlers' Farmhouse Cheeses, he loved sheep's cheese on a digestive biscuit.
He smelt some of the cheeses she had on display but did not taste any.
In the crowds outside, Steve Chester, from Washington DC, said it he was "loving the experience", of his first time seeing a British monarch in the flesh.
"Lancaster is a quaint, gorgeous town, I love it here," he added.
Lancaster University students, and "very proud Royalists" Matthew Lamb and Daniel Kirk, said they had come to "share their pride" at the Royal Family during the historic visit.
Anti-monarchy protesters were also awaiting his arrival and could be heard chanting "Not my king" as he left his vehicle.
The group had a yellow banner which said "Abolish the Monarchy" and held signs saying "Not my King" and "Ditch the Duchies".

1. King Charles III speaks with 101-year-old Richard Brock a D-Day Veteran from Lancaster
King Charles III speaks with 101-year-old Richard Brock a D-Day Veteran from Lancaster. Photo: Kelvin Lister-Stuttard. Photo: Kelvin Lister-Stuttard

2. King Charles III speaks with 101-year-old Richard Brock a D-Day Veteran from Lancaster
King Charles III speaks with 101-year-old Richard Brock a D-Day Veteran from Lancaster. Photo: Kelvin Lister-Stuttard. Photo: Kelvin Lister-Stuttard

3. King Charles III meets with members of the public outside Lancaster Castle
King Charles III meets with members of the public outside Lancaster Castle. Photo: Kelvin Lister-Stuttard. Photo: Kelvin Lister-Stuttard

4. King Charles III smells some cheese inside Lancaster Castle during his visit to Lancaster
King Charles III smells some cheese inside Lancaster Castle during his visit to Lancaster. Photo: Kelvin Lister-Stuttard. Photo: Kelvin Lister-Stuttard

5. King Charles III meets with members of the public outside Lancaster Castle
King Charles III meets with members of the public outside Lancaster Castle. Photo: Kelvin Lister-Stuttard. Photo: Kelvin Lister-Stuttard

6. Members of the public wave goodbye to King Charles III
Members of the public wave goodbye to King Charles III. Photo: Kelvin Lister-Stuttard. Photo: Kelvin Lister-Stuttard