In what was his first race over this distance since shifting his concentration from the track to road events, the four-time Olympic champion managed to finish in two hours, six minutes and 21 seconds.
It meant Sir Mo comfortably beat the two hours, seven minutes and 13 seconds achieved by Steve Jones back in 1975.
His performance was all the more impressive considering the conditions, with the 23.2C (73.8F) recorded by the Met Office at St James’ Park making it the hottest London Marathon on record – beating the 22.7C (72.9F) recorded in 1996.
Sir Mo held was driven on towards the end by some of the 800,000 spectators who lined the 26.2 mile course, cheering him on as he reached the finish line.
He was not the only Briton with reason to celebrate, as David Weir won his eighth London Marathon men’s wheelchair title.
Elsewhere, Vivian Cheruiyot, from Kenya, won the women’s elite race and Australian Madison de Rozario won the women’s wheelchair title.
Kenyan Eliud Kipchoge, who won gold at the 2016 Olympics, triumphed in the men’s elite race with a remarkable time of two hours, four minutes and 16 seconds.
The sweltering conditions had forced organisers to advise runners to rethink their fancy dress plans and drop their target finish times, with more ice, water and run-through shower stations positioned along the route.
Among the 40,000 runners taking part for charity are firefighters who tackled the Grenfell Tower blaze and a police officer who was stabbed during the London Bridge terror attack.
Members of the Stephen Lawrence Charitable Trust have also taken part on what is the 25th anniversary of the teenager’s murder.
They set off from Blackheath at 10am after the Queen started the 38th annual event from Windsor Castle by pressing the traditional red button.
The course takes runners along both sides of the Thames, with the finish line on The Mall near Buckingham Palace.
Prince Harry, who is patron of the London Marathon Charitable Trust, is in the capital to cheer people on.
More from UK
Social media firms must protect children – or face new laws, says Jeremy Hunt
Body found on uninhabited island in search for missing Somerset man
Killer whales spend day swimming in River Clyde
Clothes-munching moths more likely in warmer southern England
Stars like Tom Jones and Kylie Minogue shine for Queen’s 92nd birthday party
Two more violent deaths in London bring total to at least 58 this year
Organisers hope another record to tumble will be how much the event raises for charity, with it having registered a new high for an annual one-day fundraising event last year with £61.5m.
The annual race has raised £890m in total since 1981 and continues to grow in popularity, with 386,050 people applying to take part this year – almost a third more than in 2017 and the highest number for any marathon in the world.