The ex-European commissioner is to provide advice on business strategy to both the company and its range of clients, the firm explained.
His appointment follows the creation of strategic advisory firm Global Counsel, of which he is chairman, earlier this month.
William Rucker, chief executive at Lazard London, said: âLord Mandelson has a vast knowledge of international business matters and an extraordinary range of contacts in both the commercial and political worlds.â
Lord Mandelson said: âLazard is one of the great global advisory firms. I am looking forward to working alongside the Lazard teams in London, Paris, New York and the firmâs other offices, as well as its network of senior advisers.
âI hope that my experience in supporting trade and investment opportunities across the globe will enable me to offer real value to the business and its clients."
He held four different positions in the British government when the Labour Party was in power and worked under both Tony Blair and Gordon Brown.
By Jim Ottewill