The confectionery giant contacted at least six agencies inviting them to send in credentials and Freud won after competitive pitch. The account is thought to be broader than the public affairs and corporate remit handled by Grayling.
The firm owns brands including Snickers, Galaxy and Twix.
Grayling had held the account spanning corporate affairs, public affairs and trade PR since 1984.
The review was triggered by the recent arrival of Alison Clark as director of corporate affairs. She joined Mars from News International, where she was director of business development. Clark did not respond to PRWeek calls on the hire and Freud declined to comment.
Mars has just completed an £11.5bn takeover of chewing gum group Wrigley, making it the largest worldwide confectioner. The merged Mars-Wrigley dominates global confectionery with a 14.4 per cent market share, ahead of Cadbury with 10.1 per cent and Nestlé with 7.7 per cent, according to Euromonitor.