Matthews, 41, got down on one knee and proposed to Middleton, 33, in July 2016 in the picturesque Lake District of northwest England.
The pair first dated in 2012 before subsequently splitting — with the party-planning author later dating Nico Jackson for three years.
Three months after Middleton and Jackson parted ways, Matthews was spotted by Middleton’s side during a St. Barts vacation with her brother, James Middleton.
Matthews manages a hedge fund and is chief executive of Eden Rock Capital Management Group, which he established more than 10 years ago.
After the church ceremony, around 300 guests will travel to the nearby home of the bride’s parents, Michael and Carole Middleton, for the glamorous reception within a giant glass palace.