When James Episcopou, 22, got down on one knee to Laura Taylor the moment was captured by a host of photographers.
The couple, from Epping Forest, Essex, got engaged moments after the Mayor of London cut a ribbon at the opening of the highest vantage point in western Europe.
Mr Episcopou, a supervisor at John Lewis, said: “I’ve had it planned since July last year. When tickets first came out I thought I’ve got to buy them, it’s the perfect place to propose.
“Laura means everything to me and I wanted to make her feel on top of the world.
“We had no idea, but when we arrived we saw Boris Johnson and there were cameras everywhere.”
When asked if the presence of the media made him even more nervous, he replied: “There was no doubt I was going to do it.
“For Laura it’s made the day even more special. When people plan a proposal they think ‘how can I make it special?’. I managed to do quite a good job.”
Miss Taylor, a 22-year-old accountant, said: “I had no idea he had planned it.
“James asked me to come over to him because some photographers would like to take our picture. The next moment he was on one knee and proposing. I was so shocked. I said ‘yes, definitely’, but then I was speechless.”
The proposal took place on level 72 of the Shard, 800ft (244m) above the capital.
Mr Johnson had earlier described London’s tallest building as “a most amazing place”.
He said: “I don’t think there’s anything in London like this. You can see all the bends in the river, you can see my office, you can see Buckingham Palace, you can see for 40 miles around.”
Some 4,800 people paid for admission yesterday – at £24.95 for adults and £18.95 for children.