Little is known of his early life save that his father helped build the Grand Canal and John himself may have learned the elements of engineering at the Dublin Society's School of Drawing.
In 1806 he joined forces with two fellow engineers, Bernard Mullins and David Henry, to work on the Grand Canal. Seven years later, their firm won the contract to build the final link between the Royal Canal and the River Shannon. By then, MacMahon had married a daughter of John Killaly, the eminent engineer in charge of the Royal Canal extension; her name is not recorded and a contemporary maintained she was illegitimate. Tragedy blighted the MacMahon family in 1819 with the loss of four young children to a measles epidemic.
the 1820s and 1840s, MacMahon showed his mettle on a remarkable array
of engineering projects such as the Grand Canal, the Royal Canal and the
Custom House Docks in Dublin. He also oversaw drainage operations at
Lough Neagh and Corrib, while his bridges include the Whitworth
Aqueduct, which carries the Royal Canal over the River Inny at
Abbeyshrule, County Longford. In England he found fame by laying the
Lancaster and Preston Junction railway line.
Whitworth Aqueduct on the Royal Canal, designed by John Killaly and executed by John MacMahon
MacMahon was Engineer to the Board of Public Works when, just over 170 years ago, he produced a feasibility study for a major drainage project to 'unite the waters of Lough Erne with those of the River Shannon and the town of Leitrim.' This laid the basis for the Ballinamore and Ballyconnell Canal, later to become the Shannon-Erne Waterway.
Work on the canal began in 1846, just as the Great Famine got underway. Ballinamore itself presented a challenge becasue, as MacMahon noted, the Yellow River was so 'choked up' and 'tortuous,' that 'during every heavy fall of rain, a large extent of meadow and arable land is flooded, to the great injury of the hay and corn crops.'
Notice in a local newspaper, 1846
Poor health obliged MacMahon to step down three years later. The project was completed by the entrepreneurial engineer William T Mulvaney and ultimately cost more than twice MacMahon's original estimate of £100,000.
Following MacMahon's death in 1854 aged 73, the Institution of Civil Engineers of Ireland applauded his role in creating some of 'the most extemsive and important works' in Ireland.