Peter McMahon was born in 1867 in Tyrone, Northern Ireland. He married Margaret Daly in 1900. They had five children during their marriage.
He died on November 7, 1935, in Tyrone, Northern Ireland, at the age of 68.
Early in his life, he lived in the United States possibly training to be a mining engineer. Details are scant. Peter worked in the mining industry in Durham and eventually died of emphysema, a common industrial disease. His son Eugene was present at his death.
In the 1901 census, he was registered as a farmer but on his death certificate he was a miner.