Keitany set a new world record for a women's only marathon of 2hr 17min 01sec, smashing a 14-year-old record.
Keitany, recording her third London win to put alongside her trio of New York victories, set a new world record for a women's only marathon of 2hr 17min 01sec, smashing the 14-year-old mark of British great Paula Radcliffe.
"It was a great day for me. It was really amazing to run the best time," 35-year-old Keitany told the BBC.
"The weather was good at the beginning, it was nice for me and my pace. I've run my best," added Keitany, who saw off an Ethiopian great in Tirunesh Dibaba.
Keitany's fellow Kenyan Wanjiru, 24, recorded his greatest victory as he kept Kenenisa Bekele at bay despite a late rally from the Ethiopian legend and race favourite.
"We helped each other from the start, talking and we tried to maintain that pace for half of the race," Wanjiru told the BBC of his duel with Bekele.
Wanjiru hinted that he could see himself breaking the world record in the future.
"It was very comfortable (to be in world record pace)," he said.
Aside from the professionals running for prize money, the great majority of the 40,000 participants were amateurs running their hearts out for charitable causes.
Jackie Scully perhaps outdid the lot by getting married in the early morning on the famed Cutty Sark ship and then the 35-year-old ran in her wedding attire -- designed by the wife of former England goalkeeper David Seaman -- with her dad Eamonn in aid of a breast cancer charity.
She was diagnosed with the illness in 2014.
The runners were sent on their way by Prince William, his wife Kate and his brother Prince Harry, whose Heads Together campaign wants to remove the stigma surrounding metal health issues.
Heads Together was this year's principal charity for the marathon, which took place under blue skies for much of the time.
Heavy security measures were in place in the British capital.
And there was an extra poignancy as the runners swept round the corner of the Embankment past Westminster Bridge, scene of last month's terror attack which killed four people, while a policeman died of stab wounds within the nearby parliament grounds the same day.
Wanjiru's feat was even more remarkable given the strength of the opposition: aside from 34-year-old Bekele there was Eritrean world champion Ghirmay Ghebreslassie and Ethiopia's Olympic silver medalist Feyisa Lilesa.
Wanjiru came home in a time of 2hrs 5min 48sec to beat Bekele, who recovered from a poor patch mid-race to come roaring back in the final eight miles, but ultimately fell just short.
Bekele finished nine seconds adrift whilst Wanjiru's compatriot Bedan Karoki was third in 2hr 07:41.
Bekele, whose training was interrupted when he was ill three weeks ago, went through a bad patch and looked like he folded completely shortly after the halfway mark.
Bekele set off in pursuit of Wanjiru and in stark contrast to his downcast look earlier in the race, raised his arm in salute to a coterie of Ethiopian supporters as the race headed towards its climax.
The shock of seeing Bekele gaining on him appeared to refocus Wanjiru and he extended his lead late on, consoling Bekele with a warm cuddle when the latter crossed the line.
The wheelchair races saw a seventh win for the remarkable Briton David Weir and a maiden triumph for Swiss woman Manuela Schaer, to add to her Boston title earlier this month.