Kosgei clocked 2hr 18min 58sec, crossing the line ahead of American Sarah Hall, who timed a personal best of 2:22.01.
Kenya’s world champion Ruth Chepngetich rounded out the podium, just 4sec adrift of Hall.
Originally due to be run in April, the London Marathon was rescheduled to October because of the coronavirus pandemic that has seen many top track and field events either cancelled or postponed, most notably the Tokyo Olympic Games.
Rather than the usual mazy run around the streets of the British capital, the course this time around featured 19 1.5-mile loops around St James Park, with the iconic Buckingham Palace framing the finish line.
Kosgei, the women’s world record holder in the marathon and also twice a winner of the Chicago Marathon, cut loose from Chepngetich on the 18-mile mark and kept her rhythm as she lapped many backmarkers en route to the finish line.
“The weather is not good. So we struggled,” Kosgei said of rainy, cold conditions.
“It’s wonderful to race,” she told BBC. “We have not prepared well due
to the pandemic. I struggled up to the moment I finished.
“I will be well prepared for good results next year.”