Marwood, who takes over from Mike Greene, will be joined on the board by new vice chairman Jonathan James, an independent retailer who owns five stores in Cambridgeshire.
Marwood said he was delighted to have been elected and paid tribute to outgoing chairman Greene. He said: "We have a great opportunity to drive forward ACS as a forceful, balanced and credible voice for local shops in government and politics at all levels. It's our job to ensure that those national, local and European decision makers are convinced about the value that local shops bring to the economy, to employment and to the communities they serve."
Marwood was also celebrating this week after becoming the first convenience sector winner of The Grocer Cup for outstanding business achievement.