The Queen’s grandson, who is sixth in line to the throne, will pledge to love, comfort, honour and protect the former actress as they become husband and wife in the historic surrounds of St George’s Chapel in Windsor Castle.
Hours before the ceremony, Buckingham Palace announced that Harry has been given a dukedom - the highest rank in the British peerage - and Meghan will become Her Royal Highness the Duchess of Sussex.
Harry also received Scottish and Northern Irish titles, becoming the Earl of of Dumbarton and Baron Kilkeel, which means Meghan will become the Countess of Dumbarton and Baroness Kilkeel.
Tens of thousands of royal fans have begun to descend on the Berkshire town to catch a glimpse of the bride and groom, as police have effectively created a ring of steel around the castle in a massive security operation.
Passengers on the first train out of London Waterloo to Windsor included four women who work as cabin crew for a Canadian airline who said they wanted to be “part of history”.
“It’s a once in a lifetime opportunity. We are sending our love from Canada,” Cara Bonhage added.
Weather forecasters are expecting wall-to-wall sunshine, meaning the newlywed couple will be able to ride through Windsor town centre in the open top Ascot landau for their carriage procession.
Some 250 members of the armed forces will take part in the pomp and ceremony of the celebrations, with regiments closely connected to former soldier Harry lining the streets.
Meghan will emerge from the hour-long televised ceremony, which begins at noon, as an HRH and a member of the British royal family after saying her vows.
On Friday evening, a relaxed-looking Harry said he felt “great” as he greeted well-wishers in a surprise walkabout with best man the Duke of Cambridge outside Windsor Castle.
The pair then headed to exclusive Coworth Park country house hotel in Ascot, where the prince was spending his last night as a single man away from his bride as is tradition.
Meanwhile 15 miles away, Meghan gave a radiant smile and said she was feeling “wonderful” as she arrived with her mother Doria Ragland at the luxury Cliveden House Hotel in Taplow, Berkshire.
Around 30 members of the royal family including the Queen and the 96-year-old Duke of Edinburgh, who is well enough to attend after undergoing a recent hip operation, will gather in the 600-strong congregation.
The Archbishop of Canterbury, who will marry Harry and Meghan, has spoken of the strong bond the couple share.
“There’s profound affection between them which is wonderful and a deep understanding and mutual support,” he said.
“You see in the way they respond to each other, caring for each other, which is something you see in many couples but not always.”
As the Order of Service was published on the morning of the wedding, it revealed Harry will wear a ring - not a given for royal men - and Meghan will not promise to obey her husband in the vows.
The moving, soul classic Stand By Me by Ben E King will be performed by Karen Gibson and The Kingdom Choir, with the gospel singers also delivering Etta James’ uplifting version of Amen/This Little Light of Mine - a favourite in African-American churches.
The presiding bishop of the American Episcopal Church Bishop, Michael Curry will give the address.
Celebrities including Meghan’s former Suits co-stars Patrick J. Adams and Gabriel Macht are expected to be among the guests, along with the couple’s trusted inner circle of friends.
The Prince of Wales will take on the role of the absent father of the bride Thomas Markle, and walk his future daughter-in-law through the Quire up the altar to meet her groom.
There will be sadness amid the joy for Meghan with her father missing her big day.
The reclusive former TV lighting director will be thousands of miles away in Mexico after pulling out of attending on medical advice with just two days to go after undergoing heart surgery.
Harry too will be thinking of his late mother Diana, Princess of Wales, who died in car crash when he was 12.
He has invited her three siblings, Earl Spencer, Lady Sarah McCorquodale and Lady Jane Fellowes, as a way of honouring her.
Lady Jane is giving a reading.
Meghan will leave her country hotel with her mother to be driven to Windsor Castle, giving the first glimpse of the bride in her wedding gown.
She will walk through the Nave of the stunning chapel on her own, followed by a procession of bridesmaids and pageboys, before meeting Charles at the start of the Quire.
William, whose children Prince George and Princess Charlotte will be among the young helpers, will be supporting his younger brother Harry at the altar.
The Duchess of Cambridge will be there, making her first public appearance since giving birth to her third child Prince Louis less than four weeks ago.
Meghan, who found fame played Rachel Zane in the US legal drama Suits, is the first mixed race person in modern history to marry a senior British royal.
Harry, 33, and 36-year-old Meghan became engaged following a whirlwind 16-month romance after going on a blind date in London.
Meghan has been married before.
She wed first husband film producer Trevor Engelson in 2011 but the pair divorced less than two years later.
The last time a senior royal wanted to marry an American divorcee, it sparked a constitutional crisis and rocked the British monarchy to the core.
King Edward VIII, Prince Harry’s great-great-uncle, abdicated over his love for American divorcee Wallis Simpson in 1936.