Dowdney runs two c-stores in Norfolk and is chairman of Buy Local, a campaign to encourage shoppers to support local independent businesses. He also played a key role in a campaign to stop Tesco's expansion in Stalham.
He replaces Mike Sherwood, who is stepping down from the role but will remain involved in ACS committee work.
"I am a fervent believer in independent and small businesses and being on the board will allow me to put my weight behind the work ACS does," said Dowdney.
ACS CEO James Lowman added: "Nigel is a shining example of independent retailing in the UK. He has a broad understanding of all the issues that our members face."