Rosemary – who married Andrew in 1996, the year after his divorce from first wife Camilla, the Duchess of Cornwall, was finalised – died on January 10, aged 69, after battling cancer for several years.
Her death came just two weeks after Andrew and Camilla's daughter, Laura Lopes, gave birth to twin boys, Louis and Gus.
"She was very pleased to have made Christmas and then Andrew's birthday, his 70th, just after Christmas, and subsequently the birth of his grandchildren," said Henry Pitman, one of Rosemary's three children from her first marriage.
Other guests paying their respects at the service at St Aldhem's Catholic Church, Malmesbury, included Princess Anne's husband Tim Laurence, and Andrew and Camilla's two children, Laura and Tom Parker Bowles.
Camilla has stayed close to her former husband since their divorce. Former army officer Andrew and garden designer Rosemary have joined her and Prince Charles at Sandringham on various occasions.
Andrew is also a close friend of the Princess Anne, and dated her in 1970.
A thanksgiving service for Rosemary will take place in London in March.