The couple, both 28, who have been dating on and off for about eight years, lit up the English countryside just a day after getting back from a safari break in Kenya, Africa.
Middleton appeared every inch the princess-to-be in a bright blue dress, fitted black jacket, sky-scraping heels and a huge hat adorned with feathers.
"She looked so happy," an observer tells PEOPLE.
The prince was dapper in conservative tailcoat and purple tie, smiling for fans and photographers as he arrived with Middleton at the church in Northleach.
"They were both so relaxed, and she looked incredibly demure by his side," adds another observer, royals author Judy Wade. "She was very much his fair lady in her big picture hat. You get the impression it's the sort of wedding they would wish they could have – in Gloucestershire with friends, rather than a big formal wedding in central London."
The couple were at the service for friends Harry Meade and Rosie Bradford at St. Peter and St. Paul church in Northleach, Gloucestershire. At the end of the ceremony, the pair walked away from the church to give the waiting photographers a clear shot.
Might a royal wedding be next? For now, the British heir and his love are happy to keep everyone guessing.