Roderick Urquhart, of Samuelston, has been appointed to the role by Her Majesty The Queen.
He was previously named a Deputy Lord Lieutenant in March 2016 alongside Pauline Jaffray.
The Lord Lieutenant is the Queen’s representative in East Lothian. His deputies are there to step in when he is unavailable.
Mr Williams retires from the post on March 15 after eight years’ service.
Lord Lieutenants and their deputies must retire when they reach the age of 75.
Mr Urquhart said of his new role: “It is a great honour and a privilege to be appointed as Her Majesty’s Lord Lieutenant of East Lothian, and to succeed Michael Williams MBE.