"We were out there with friends, so I really didn't expect it at all," Middleton said. "It was a total shock when it came. Very excited."
The announcement came to the elation of Britons, many of whom viewed the engagement as a seemingly inevitable yet unreachable moment. Within 25 minutes of the announcement's posting on the royal Facebook page, more than 1,400 people "liked" the news. The numbers have since increased.
"It's about time! Congrats," one commenter wrote, a sentiment echoed by hundreds of others.
The two met nearly a decade ago while students at the University of St. Andrews in Scotland. The relationship garnered some controversy because Middleton does not come from royalty and royals are generally expected to wed other aristocrats.
William said the two were friends for more than a year and lived as flatmates. "It blossomed from then on," he said. "We had lots of fun. ... Same interests. She has a really naughty sense of humor."
When asked why it took so long to propose, a beaming William quipped, "I also didn't realize it was a race, otherwise I probably would've been a lot quicker.
"We have a great fun time together," the prince told reporters. As for the ring, William said, it, like Kate, was very special to him.
"Well as you may recognize now, it's my mother's engagement ring and it's very special to me, as Kate is very special to me now as well. It was only right the two were put together," William said. "It was my way of making sure mother didn't miss out on today and the excitement and the fact that we're going to spend the rest of our lives together."
Diana received the ring from Prince Charles after the two were engaged in 1981. At the time, the sapphire ring, which is surrounded by 14 diamonds, cost 30,000 pounds, or about $47,500 at today's conversion rate.
"I had been carrying it around in my rucksack for about three weeks before [the proposal]," William told ITV. "Everywhere I went, I was keeping ahold of it, cause I knew if this thing disappeared, I'd be in a lot of trouble."
"It's beautiful," Middleton said, as she showed the ring to ITV's camera.
While Middleton is now wearing Diana's ring, William said, she won't be in his mother's shadow.
"There's no pressure," he said. "Like Kate said, it is about carving your own future. No one is trying to fill my mother's shoes. What she did was fantastic. It's about making your own future and your own destiny, and Kate will do a very good job of that."
William said he didn't know where to start when asked why the couple was apparently so loving, but said she makes him laugh. For her part, Kate said William was very supportive of her.
William and Kate broke up briefly in 2007 but were back together within months.
Prince William is second in line for the throne, after his father Charles, Prince of Wales. Prince William's mother was Diana, Princess of Wales, who died in an auto accident in 1997.
The royal couple plan to live in north Wales, where Prince William will continue his service in the Royal Air Force, the statement said.
ABC News' Nick Watt, Sarah Netter, Jason Volack and Bradley Blackburn contributed to this report.