Coulson is due to go to trial next year on charges related to his alleged involvement in the phone hacking scandal at the News of the World.
He served as editor of the now defunct tabloid between 2003 and 2007. He later became the Conservative Party's director of communications.
Last year, the High Court ruled that NGN was not liable for Coulson's legal fees as he fights charges related to alleged phone hacking, and also illegal payments made to police.
But Coulson - who denies any wrongdoing - sued the media group, seeking a declaration that they "must pay the professional costs and expenses properly incurred" by him "in defending allegations of criminal conduct" during his time at NGN.
NGN had stopped paying his legal costs in August 2011, saying that a clause in his contract covered the "occupational hazards of being an editor" but not alleged criminality.
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However, Lord Justice McCombe, sitting at the Court of Appeal in London, allowed Coulson's appeal.
The judge said that he was "satisfied" that a clause in Coulson's contract with NGN did cover "costs and expenses of defending the criminal allegations".
NGN has not yet indicated whether it will make a further appeal to the Supreme Court.