Murphy, who was global CEO of the company for four years, became non-exec chairman in February when Pete Pedersen succeeded him as CEO.
He has had a long working relationship with Huntsworth chief exec Lord Chadlington, having worked together in the 90s at Weber Shandwick before becoming CEO Trimedia International, which was sold to Hunstworth and later merged with Mmd and Grayling.
Murphy commented: ‘Grayling is in good hands with Pete and his strong management team and I am keen to pursue multiple opportunities both in marketing communications and other areas of business.’
‘I’ve been an entrepreneur all my life. I’ve started and built businesses and I can see a lot of opportunities in the market place. I want to start with a blank canvas.'
Lord Chadlington added: 'Michael has had a long and distinguished career first at Shandwick and now at Grayling where he brought together all our brands under one banner with great consideration and care. We all wish him the very best in his pluralist future.’
Last week Hunstworth announced that Grayling's first half revenues slid by 7.2 per cent to £40.4m, contributing to a 0.5% like-for-like drop in parent company Huntsworth's overall revenues.