Van Straubenzee will marry party and wedding videographer Daisy Jenks this afternoon, on the same day that the Duchess of Sussex celebrates her 37th birthday.
The Duke of Sussex arrived first, accompanying Charlie into St Mary the Virgin Church in Frensham, Surrey, as he assumed best man duties.
Prince Harry, who has been close to van Straubenzee since childhood, wore sunglasses as he stepped into the building among a group of groomsmen.
Charlie was an usher at Harry's recent wedding, along with older brother Thomas van Straubenzee.
Meghan, who is celebrating her 37th birthday today, waved to well-wishers and briefly chatted among other guests outside the church when she arrived.
She wore a navy blue short-sleeved dress and matching fascinator.
William is good friends with the van Straubenzee brothers, with Thomas a godfather to his daughter Princess Charlotte.
Last year Harry reportedly whisked Meghan away for a romantic break in Africa to mark her 36th birthday, flying to Botswana.
She married Harry during a ceremony watched by an estimated television audience of hundreds of millions, and it was praised for combining tradition with modernity and diversity.
Sir Elton John, who performed at the wedding reception, described it as a memorable event and said: "To have a black gospel choir, a multiracial bride, a black cellist, who was wonderful, a preacher - it felt like a party, and it felt like progress had been made.
"Thank God, thank God."
Sir Elton added: "I think the Queen had a lot to do with it. I think she's been magnificent. It was a wonderful thing to be at."
Harry and Meghan were able to enjoy their honeymoon without being discovered, jetting off to their undisclosed destination soon after they wed on May 19.
They live in a property in the grounds of Kensington Palace but are rumoured to also have a country bolt-hole somewhere in the Cotswolds.