The Duke and Duchess of Cambridge were at the showing on London's Leicester Square, joined by 600 serving and ex-serving military personnel and their families, as well as beneficiaries of military charities.
The royal couple attended the event in aid of the Foundation of Prince William and Prince Harry.
The movie's star, thoroughbred Joey, was driven from his stables in Milton Keynes to face fans and photographers as he trotted down the red carpet.
Spielberg said he is "delighted" that the premiere was being held in aid of the royal brothers' foundation.
"War Horse encapsulates for me a timeless personal story of courage," said the director. "The movie, which I am proud to say was filmed here in Great Britain, deals with the discovery of humanity against conflict, and celebrates bravery, loyalty, tenacity and hope.
"We are honoured to dedicate this event in aid of the Foundation which recognises the welfare of those who serve their country in the British Armed Forces."
The First World War epic, which has been nominated for best drama at the Golden Globes, is based on a play inspired by children's author Michael Morpurgo's novel.
It stars Sherlock actor Benedict Cumberbatch, Emily Watson, Tom Hiddleston and David Thewlis and follows on from the success of the play which debuted at the National Theatre on London's South Bank in 2007 before sell-out West End and Broadway runs.
SLIDESHOW: See the stars on the red carpet...