He joined the Irish Civil Service shortly after graduating from Trinity College and served in the Department of Industry and Commerce.
[3] He volunteered at St. Andrew's Resource Centre on Pearse Street, Dublin while attending Trinity College.
McCarthy was close to Taoiseach Brian Cowen on partnership and other issues and often accompanies him on official engagements in Ireland.
In April 2011 after the 2011 general election it was announced that McCarthy intended to step down a few months ahead of schedule as part of a major reorganisation of the Civil Service.
He also announced the government had decided to create a new post of second secretary in the Department of the Taoiseach to support the Economic Management Council established by the Coalition.