Mary Morton, a 99-year-old retired assistant principal in the Bronx, N.Y., wanted to live long enough to watch Melanie Renee White, her granddaughter, walk down the aisle with her then longtime boyfriend, Andrew R. Trotter.

“I hope my eyes are still open by the time y’all get married,” she told Ms. White while the couple was dating.

On Feb. 22, Ms. Morton’s eyes were fixed on her granddaughter, 37, and Mr. Trotter, 38, as she led their ceremonial celebration at Secret Gardens Miami, an event space in Homestead, Fla.

“She just wanted to see it happen so bad,” said Ms. White, who had legally married Mr. Trotter on Jan. 18 at Philadelphia City Hall. “It was just an honor to have her do it.”

The couple had tested the nonagenarian’s patience: They first met over a decade before, in 2012, when Ms. White, who now runs a custom jewelry line called Melanie Marie, worked as a sales associate at Saks Fifth Avenue in Philadelphia. Mr. Trotter, who flips residential real estate, visited the store looking for a pair of 7 for All Mankind jeans. After meeting Ms. White, he left with nine pairs and his newfound crush’s phone number.

“I thought she was just beautiful,” Mr. Trotter recalled.