In a brief appearance in the Sunken Garden at Kensington Palace, Harry told reporters he knew Markle was “the one” from “the very first time we met.”
The prince declined to elaborate on his proposal to Markle, but when asked whether it was romantic, he smiled and said, “of course it was.”
Markle said she was “so very happy” and flashed her engagement ring to reporters.
The ring, designed by Harry, has a gold band and features a large diamond from Botswana and two smaller outer diamonds from the personal collection of Harry’s late mother, Diana, Princess of Wales, according to a statement from Kensington Palace.
Botswana holds special significance for the couple, who have spent time together there over the past year and a half, Kensington Palace said.
The pair, who became engaged quietly earlier this month, will marry in spring 2018 and live at Nottingham Cottage at Kensington Palace, where Harry currently lives.
Markle will be the first American to marry a British royal since the marriage of Wallis Simpson to King Edward VIII, a union so controversial that he was forced to give up the throne in December 1936 to go through with it.
Monday’s announcement was made by Harry’s father, Prince Charles, in a statement released from his office at Clarence House.
Prince Charles said he was “delighted to announce” the engagement, adding that Harry had informed “The Queen and other close members of his family.”
“Prince Harry has also sought and received the blessing of Ms. Markle’s parents,” the statement added.
The couple went before photographers on Monday afternoon before taking part in a broadcast interview which will air in the evening.
Harry’s grandparents, Queen Elizabeth II and her husband Prince Philip, are “delighted” for the couple and “wish them every happiness,” a Buckingham Palace spokesman said.
The palace did not say what titles would be conferred upon the couple when they are married, but the prince is likely to be awarded a dukedom, meaning Markle would become a duchess. Markle will not become a princess in her own right: only daughters of royals can claim such status.
Shortly after the announcement, Kensington Palace released a statement on behalf of Markle’s parents, Thomas Markle and Doria Ragland.
“We are incredibly happy for Meghan and Harry. Our daughter has always been a kind and loving person. To see her union with Harry, who shares the same qualities, is a source of great joy for us as parents.
“We wish them a lifetime of happiness and are very excited for their future together,” the statement ended.
Harry’s brother, William and his wife, Catherine, who also reside at Kensington Palace, shared their joy at the impending union on Monday.
“We are very excited for Harry and Meghan. It has been wonderful getting to know Meghan and to see how happy she and Harry are together,” they said in a statement.
Monday’s announcement ends persistent speculation over when the 33-year-old prince – who is now fifth in line to the British throne – would propose to the actor.
Harry and his fiancée made their first public appearance as a couple in September at the Invictus Games in Toronto.
The pair were introduced in July 2016 by mutual friends in London, Markle confirmed in September. Markle is best known for her role on TV show “Suits” as well as her philanthropic work.
The 36-year-old LA native spoke of her royal relationship for the first time earlier this year telling Vanity Fair: “We’re two people who are really happy and in love.”