Edward Mahony was born at Ballincollig, County Cork, Ireland, probably in 1824 or 1825; his parents' names are unknown. As a young man he was apprenticed to his uncle, John Mahony, an architect and builder in Cork, a city which had attracted a number of prominent Gothic Revivalist architects. He married Margaret Barry, probably in 1848 or 1849.
In 1854, apparently disillusioned by the lack of opportunities for Catholic architects in Ireland and by the hardship caused by famine and plague, Edward Mahony, his wife and the first two of their 11 children sailed on the Telegraph for Adelaide, South Australia, arriving in January 1855. At the end of the year, the family continued on to New Zealand, arriving in Auckland in February 1856. A son, Thomas, was born at sea on 12 December during one of these voyages, probably the first. Edward, having changed the spelling of his name to 'Mahoney' to avoid confusion with an Auckland solicitor named Edmund Mahony, set up in business as a builder and timber merchant.
Mahoney was again engaged in architecture by 1861, when he designed the Church of St John the Baptist, Parnell, using a pared-back Gothic style. This, and St Mary's Convent Chapel, Ponsonby (1866), are notable for the plainness of their well-lit interiors and the use of cross-braced roof trusses.
Attracted by the discovery of gold on the Coromandel Peninsula, Mahoney spent the years from 1867 to 1870 in Thames before returning to Auckland, where in 1870 he set up an architectural practice. He designed St George's Anglican Church, Thames (1871); St Columba's Presbyterian Church, Warkworth (1876); Holy Trinity Church, Dargaville (1878); and St Andrew's Church, Cambridge (1881).
In 1876 Thomas Mahoney joined the practice, which became known as E. Mahoney and Son. They produced many of Auckland's banks and hotels during the boom of the 1870s and early 1880s, as well as most of its Catholic schools and churches. Edward Mahoney prospered and was able to build a large house in Harbour Street, St Marys Bay, staffed with servants and boasting a carriage, coachman and horses.
Edward Mahoney's finest work is the large Anglican Church of the Holy Sepulchre, Khyber Pass Road (1879–81), built to accommodate a growing congregation in Auckland's new suburbs. Praised for its 'severe simplicity', its interior is notable for height, lightness and the warmth of its stained kauri timbers. It represented a departure in New Zealand ecclesiastical architecture, and its seven-sided apse is unique in New Zealand.
Edward Mahoney took an active part in Auckland's professional and civic affairs. He was a member of the Provincial Board of Education and in 1878 designed the first permanent Auckland College and Grammar School building in Symonds Street. He was a foundation member of the Auckland Institute of Architects formed on 23 December 1880, and became its first honorary treasurer in 1881.
In 1885 Edward Mahoney retired; Thomas and a younger brother, Robert, carried on the practice. In 1887 Thomas designed the brick St Benedict's Church, Newton; it replaced Edward's wooden original, which had been destroyed by fire in 1886. St Mary's Church of the Assumption, Onehunga, designed in 1877, was built in 1888. For some 20 years from 1905 its parish priest was Monsignor William Mahoney, another of Edward Mahoney's sons and the first New Zealand-born Catholic priest.
In 1880 Edward Mahoney had drawn up plans to extend the stone building which was the original St Patrick's Cathedral, Auckland. Between 1884 and 1885 the nave was extended according to Edward's scheme, but Thomas was ultimately responsible for its further extension, the sanctuary, four sacristies and two side chapels, which by 1907 had transformed a modest structure into a large and impressive building befitting its status as a cathedral.
Thomas Mahoney's most unusual church, and the practice's only one built in a neo-classical rather than a Gothic style, was the since-demolished Church of Our Lady of the Rosary, Hamilton (1912). He returned to a Gothic design in 1919 for All Souls Church, Devonport. This was built over his father's existing 1865 mortuary chapel, the Church of St Francis de Sales, which in 1892 had been punted across the Waitemata Harbour en route from its original Symonds Street site to the Catholic cemetery on the slopes of Mt Victoria.
Secular buildings also formed part of Thomas Mahoney's work, beginning with James Williamson's enormous Italianate house, The Pah, at Hillsborough (1877). He was also responsible for the Customhouse, Auckland, built to a French Renaissance design between 1888 and 1890; the Dilworth Terrace flats of 1900; buildings for the Bank of New Zealand throughout the Auckland provincial area; and for notable warehouses in Auckland. In 1910 he designed an impressive college in Gothic style for the Society of the Sacred Heart, Remuera; it is now known as Baradene College.
Like his father, Thomas Mahoney was involved in professional affairs. In 1907 he became president of the Auckland Institute of Architects, and in 1913–14 was president of the New Zealand Institute of Architects. By contrast with his father, who was said to have had a quiet and retiring disposition, Thomas was a sociable and cultured man, fluent in French and German. An accomplished watercolourist, he studied with J. B. C. Hoyte and was a keen recorder of picturesque places in the North Island, to which he travelled on foot. He married Charlotte Wallnutt in Auckland on 26 November 1889; they had three daughters.
Edward and Thomas Mahoney made a considerable contribution to Auckland's architectural heritage. Both died at Auckland: Edward on 28 April 1895 and Thomas on 8 September 1923. Edward's wife, Margaret, had died in 1891, while Charlotte Mahoney died in 1944. The practice was dissolved in 1926.