Philip, whose official title is the Duke of Edinburgh, was expected to celebrate in private, with no public events scheduled.
The royal family shared a photo of a then 32-year-old Philip looking dapper in a suit and tie.
The Duke and Duchess of Cambridge wished Philip many happy returns by sharing some handsome photos on Twitter and Instagram with the caption: "Wishing a very Happy 98th Birthday to His Royal Highness The Duke of Edinburgh!".
Congratulations are in order as Prince Philip is celebrating his 98th birthday on June 10, 2019. Prince Philip and Queen Elizabeth have been married since 1947-shortly after the end of World War II.
"Wishing a very Happy 98th Birthday to His Royal Highness The Duke of Edinburgh!" the caption read. "I found a couple of classic photographs of Grandpa and I on Christmas Day and Trooping the Colour a few years ago now".
The Duke of Edinburgh still drives a horse-drawn carriage on private grounds from time to time, but has stopped being behind the wheel of cars since a serious accident in January.
The sweet birthday posts come just days after the royal family officially celebrated the Queen's birthday with the annual Trooping of the Colour parade, though the monarch's actual 93rd birthday was back in April.
He was spotted in good form last month, laughing and joking with other members of the royal family at Lady Gabriella Windsor's wedding.
Philip retired from his royal duties on 2 August 2017, at the age of 96, having completed 22,219 solo engagements since 1952.
The queen and Philip have four children, eight grandchildren, and eight great-grandchildren.