Macmahon was born in Belfast and raised Catholic. He was educated at St Patrick's College (Armagh) and at 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 GPO worker Nancy O'Brien due to her dedication and purported lack of interest in politics. However, O'Brien was a second cousin of Michael Collins, and each day, between 2:30 and 3:30, she would pass any salient information she 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 went into business in Dublin. In 1928, he was elected president of the Royal Irish Society. He died on 1 May 1954, aged 89.[where?] His obituary was published in The Times on 3 May 1954.
Sir William Byrne
|Under-Secretary for Ireland
[[Category:Place of death missing]
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