MacKay, MP for Bracknell and a former parliamentary aide to David Cameron, will become an international consultant and strategic adviser in the global PR agency's UK public affairs team.
The news come days after Tory leader Cameron launched a fierce attack on lobbying, effectively blaming lobbyists for declining levels of public trust in Parliament.
MacKay announced last year that he would be standing down as an MP, along with his Tory MP wife Julie Kirkbride, after details of their expenses claims were revealed.
Mackay was instructed to repay £31,193 by Sir Thomas Legg, following his review of expenses claimed over the past five years.
The former aide to Cameron confirmed that he was entering the lobbying industry. In a statement, Mackay said: ‘After leaving Parliament I want to be a part of a winning team that is assisting global businesses face their strategic communications challenges and Burson-Marsteller provides a fantastic opportunity to do this.'
He added: 'I considered a number of consultancies and decided on Burson-Marsteller because of its range of international clients, its campaigning approach and the quality of the team.'
Earlier this week, Cameron used a major speech to lay into lobbyists. He said: ‘It's an issue that crosses party lines and has tainted our politics for too long...an issue that exposes the far-too-cosy relationship between politics, government, business and money. I'm talking about lobbying - and we all know how it works.
‘The lunches, the hospitality, the quiet word in your ear, the ex-ministers and ex-advisers for hire, helping big business find the right way to get its way.
'In this party, we believe in competition, not cronyism. We believe in market economics, not crony capitalism. So we must be the party that sorts all this out.'