The Indian-born 100-year-old from Ilford, east London, completed the full distance in Toronto more than six hours after the winner, as workers were dismantling the barricades.
His total time of 8.25.18 should make him the Guinness World Record holder once all documentation is verified.
Born in the Punjab on 1 April 1911, Singh was a farmer when he first developed a love for running, but only took it up again 11 years ago after moving to live with family in the UK following the deaths of his wife and son. He ran his first marathon aged 89, and has since run seven more.
The holder of several records for fastest nonagenarian, he also claimed eight centenarian distance records from 100 metres through to 5,000 metres during Sunday's Toronto waterfront marathon. Some had never been attempted by someone that old before.
Though Singh found he "hit the wall" at 22 miles he soldiered on for another two hours, and even finished ahead of five other competitors.
His coach, and translator Harmander Singh, said: "Just before we came around the final corner, he said that 'achieving this will be like getting married again'. He's absolutely overjoyed, he's achieved his lifelong wish."
Singh, who attributes his success to ginger curry, cups of tea and "being happy", has long been acclaimed as the "Turbaned Tornado" – the title of his biography, published last year, which he unfortunately has not been able to read because he is illiterate.
In 2004 his running prowess saw him replace England footballer David Beckham, and champion boxer Muhammad Ali as poster boy for Adidas's "Impossible is nothing" advertising campaign.
He now hopes his next project will be participating in the torch relay for the 2012 London Games.