Emma Gordon took several photos of Peterborough's Christmas tree on Thursday, and later noticed they showed two strangers' romantic moment.
Ian Peasgood came forward after seeing a Facebook appeal to trace the couple.
"It's absolutely fantastic," Mr Peasgood said. "I didn't get a photo at the time, so this is perfect."
Mr Peasgood, who lives in Leicester, had decided to pop the question to his girlfriend Anita Robson, from Whittlesey, during a late night shopping trip to Peterborough.
"I'd bought the ring 20 minutes earlier and had planned to go to the cathedral," Mr Peasgood, 45, said. "But I was drawn to the tree - it's really nice.
"I was shaking like a leaf as I asked her."
Ms Robson, 44, said yes and the couple plan to marry in 2017.
"As we walked off I thought 'I should have taken a photo of the tree so we had a momento'," Mr Peasgood said.
Five days later Mr Peasgood said his fiancee was checking her phone when she spotted a picture of the Peterborough tree.
"She scrolled up and saw us!", said Mr Peasgood, who made himself known after the Peterborough Telegraph linked to the photos.
"It's such a good photo. Anita wants it framed and on the wall.
"We'll keep it for as long as we live."
A man inadvertently captured proposing to his girlfriend has thanked the photographer, who tracked him down via social media.