Kipsang, second fastest man in the world over the distance, won in two hours four minutes and 44 seconds, just four seconds off the course record set last year by Kenyan Emmanuel Mutai.
Keitany became the third fastest woman in the marathon as she retained her title in 2:18.37 ahead of world champion Edna Kiplagat and Priscah Jeptoo.
Florence Kiplagat and Lucy Kabuu completed a Kenyan sweep of the top five women's places.
London was the final chance for the athletes to impress the Kenyan selectors with the Kenyan Olympic marathon team due to be announced at the end of the month.
Kipsang, whose personal best of 2:03.42 is three seconds off the world record, surged to the front of the men's race just after the halfway stage.
He was joined by Ethiopia's Feyisa Lilesa at around 25 km and twice world champion Abel Kirui then came though strongly to reach them at 30 km.
The trio went through to 35 km when Kipsang accelerated again and took an unassailable lead.
"I knew when I went away they would have to work very, very hard to beat me as I was feeling very good in myself," Kipsang said.
Kenyan Kirui dropped off the pace and eventually finished sixth.
Three-times London winner Martin Lel made up some ground late in the race and out-sprinted Ethiopia's Tsegaye Kebede on the home straight up The Mall to make it a Kenyan one-two.
The last few strides I was so pleased to beat him," said Lel. "When the winner went off today, I knew I didn't have it in me to go with him. I'm very pleased to get second place today.
It shows that my form is getting back to where it was."
World record holder Patrick Makau dropped out of the race at around the halway mark.
In the women's race, the five Kenyans had been in the lead group from the start.
Keitany hit the front once the pacemaker dropped out after 30 km and was shadowed by Edna Kiplagat.
At 35 km the pair had a four-second lead but Keitany's relentless pace proved too much for Kiplagat and the champion pulled away to win comfortably.
Only British world record holder Paula Radcliffe and Russian Liliya Shobukhova have run faster than the Kenyan.
"I'm pleased with my run today and the time was ok," said Keitany. "The weather here all week as not been good but when I saw the sunshine this morning, I knew the weather would be right for me," added the 30-year-old.
Britain's David Weir won the men's wheelchair race for a record sixth time.
Officials said 35,970 competitors had taken part in the marathon.