Henry Chilcott, the managing partner at Antidote, was lunching with the agency's creative director, Tim Ashton, at the top adland restaurant Quo Vadis when the building opposite caught fire.
Firefighters fought the blaze, at the former home of the production company Future Films, for most of the afternoon before bringing it under control.
Chilcott said: "I was having lunch with Tim Ashton at Quo Vadis (directly opposite) at the time the fire started.
"As we tried to leave for a client meeting, we found that the police and firefighters had locked us in."
After this good fortune, diners carried on eating, but were forced to evacuate half an hour later as the temperature inside started to rise.
Chilcott said: "Despite the efforts of a rotund policeman to induce panic, the diners calmly topped up their wine glasses before being ushered to safety through the basement kitchens."
Police closed Dean Street over the weekend while the fire brigade investigated the house, which has been described as one of the best-preserved 18th-century homes in London.