The 31-year-old from Dundee, who underwent two knee operations either side of Christmas 1993, received huge support in what was the hottest race in the London Marathon's 16-year history, with temperatures rising towards 80F.
About 3,000 runners were treated for the effects of heat.
After becoming the first British woman to win in London since Veronique Marot seven years ago, McColgan paid tribute to her coach, Norway's former world marathon champion Grete Waitz.
McColgan's victory confirmed that she is back to the kind of form which brought her the world 10,000-metres title five years ago, and provided ideal preparation for her Olympic challenge this summer.
"My rivals are going to have to do something extra special if they are going to prevent me from coming back from Atlanta with gold," she said.
"Things happen to you for a reason. I believe the problems I have faced in the last three years have been a test and I am a better, stronger person for coming through it. I always knew in myself that I could make it back."
McColgan took the lead with six of the 26 miles remaining after catching Anita Haakenstad of Norway, whose lone run threatened the upset the formbook in a race involving some of the world's fastest runners.
The men's race was won by Dionicio Ceron of Mexico who completed an unprecedented hat trick of London wins in the men's race. Paul Evans, in third place, was the first male Briton home.