The celebration, hosted by King Harald and Queen Sonja, was lead by former Oslo Bishop Gunnar Stålsett, who married Mette-Marit and Prince Haakon twelve years ago.
About 350 guests, among them Mette-Marit's childen, family, friends, and members of staff and representatives of charities and organisations that the Princess works with prepared for the wet weather at the celebration. The Norwegian royals also brought along their beloved family pet labradoodle Milly Kakao, who recently gave birth to nine puppies.
The Norwegian Princess beamed happily the day before her birthday, with one of Milly's pups in her arms. She dressed in black wellingtons and a cream Mac, with her hair loosely tied back, although luckily the sun was shining for the intimate gathering.
Mette-Marit, 40, became emotional as her nine-year-old daughter Princess Ingrid Alexandra read the Prayer of Saint Francis, which was also read during her wedding in 2001.
She wiped away tears when the King and Queen took the stand to praise and thank their daughter-in-law.
After wishing her a happy birthday, Queen Sonja said, "This celebration says a lot about you", adding that she carries out her projects with "generosity, empathy and knowledge".
Mette-Marit's husband Prince Haakon's 40th birthday party was significantly more lavish, comprising a weekend-long music festival at Skaugum Palace.
The Princess – who has been celebrating her birthday each year with a church service together with family and friends since marrying her husband – was clearly moved at the speech.
After the service, the King and Queen invited everyone, including spectators and press, for cake and coffee in the park.
Even the family’s labradoodle didn't miss the landmark birthday. Milly is always involved in special occasions, also joining the family for photographs on Norway's National Day in May. The adorable pet recently gave birth to nine puppies, and updates were posted on the royal family’s official website, showing that she is very much a part of the family.