Brendan Ogle, the Secretary of the ESB Group of Unions, is leaving ESB and his position as Group Secretary.
Mr Ogle represented staff in ESB for nearly ten years, initially as a regional organiser for Unite and more recently as Secretary of the ESB Group of Unions.
In a statement, Mr Ogle said the decision was initiated by him and was made by mutual consent.
He said the group of unions' recent successful campaign to have the ESB pension scheme reclassified and treated as a defined benefit pension scheme allows him to leave the company "at an appropriate time".
In a statement, the ESB said it wished Mr Ogle well in his future endeavours.
"While 2013 was a difficult period for industrial relations in ESB and in turn for customers, ESB acknowledges that over the period of his involvement with ESB as a union official and as Secretary of the ESB Group of Unions, Mr Ogle made a significant contribution, particularly through the delivery and implementation of a number of important change agreements," the company added.