The actress - who plays Margaery Tyrell in the HBO series - ran the 26.3 mile course in aid of children's charity Barnardos.
London Marathon: Which celebrity ran faster (and did they beat DS)?
Dormer revealed on her fundraising page that she has been training for the event while shooting Hunger Games: Mockingjay - in which she stars as Capitol film director Cressida - in Atlanta.
Olympian Mo Farah also took part, finishing his first ever competitive London Marathon in eighth place with a time of 2 hours 8 minutes.
I'm a Celebrity... Get Me Out of Here! contestant Amy Willerton, former Made in Chelsea star Hugo Taylor and retired footballer Michael Owen were among the other famous faces running today.
The former England player took to Twitter after the race and said that he was "relieved" to have finished the course in a time of 3 hours 45 minutes.
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Pleased with my time of 3:45 but the pain during the last dozen miles has put me off for life! So grateful for all those who donated.
— michael owen (@themichaelowen) April 13, 2014
He added: "The support of the crowd and other runners was like nothing I've seen before. Such a good atmosphere and so relieved its over!"
Also taking part were Miquita Oliver - who ran for Oxfam - Dominic West and Jenni Falconer.
Falconer crossed the finish line in 3 hours 57 minutes, tweeting: "Well that was the toughest run ever. But I finished! Woop! 3:57. #LondonMarathon."
See the celebrities in action in our Marathon gallery below:
