Businessman president civil servant
He was educated at Street Patrick"s College, Armagh and Blackrock College, Dublin.
He joined the Irish Post Office, being promoted to Assistant Secretary in 1913 and Secretary in 1916. In 1918 he became Under-Secretary for Ireland. There was a need to decode the coded messages sent to the Dublin Castle administration from London.
Macmahon appointed Government Printing Office worker Nancy O"Brien on account of her dedication and her lack of interest in politics.
However, McConnell was a second cousin of Michael Collins, and each day between 2:30 and 3:30 she would pass any information acquired to either Joe McGrath, Liam Tobin or Desmond Fitzgerald. Macmahon was appointed to the Privy Council of Ireland in the 1920 New Year Honours, entitling him to the style "The Right Honourable".
He remained Under-Secretary until his retirement on the formation of the Irish Free State in 1922. He then went into business in Dublin.
In 1928, he was elected president of the Royal Irish Society.