The husband-and-wife team have run the UK agency's creative department since 2002, when it was still known as IMP.
Mills and Nolan have worked together for more than a decade. They were at Impiric, now Harrison Troughton Wunderman, for two years. They joined from OgilvyOne, where a five-year stint saw them rise to creative director level.
The team has one of the best creative reputations in London. Since they took over at Arc, the agency has won 60 awards and nominations for its creative work, including a gold and two silvers for Fiat and a silver for Phillips at the DMA Awards in December.
The agency has not yet decided on their replacements. However, their deputies, Garry Munns and Aaron Martin, will take over in the interim.
Mike Spicer, the managing director of Arc UK, said: "We are very sad to be losing Graham and Jac. They have been an important part of our team since they joined in January 2002. We wish them all the best for their future."