Gannon was born in Sydney, New South Wales in 1847, the son of James Gannon and his wife Mary. After working as a clerk at the Christian Brothers' College Sydney and as a commercial agent with his brother, he relocated to Queensland in 1868. He acquired pastoral experience at Warra Warra and acted as manager for Thorn and a stockbuyer for Davenport. In 1880 he became an auctioneer in Ipswich. In partnership with R.A. Ryan, he purchased the produce and auctioneering company of Arthur Martin in 1882. He invested in grazing and real estate. On 6 June 1884, he married Amy England Pearce in Brisbane.
The Gannon's permanent residence was at "Waratah", Toorak Road, Breakfast Creek. In February 1888, Michael Gannon purchased a large portion of land in the Wynnum-Manly area formerly part of Portion 57. Gannon sold off a considerable acreage of land, but retained ownership of that portion on which a residence was to be constructed. It is likely that the house was constructed as, on 30 April of that year, a Bill of Mortgage was registered from Michael Brennan Gannon to the Royal Bank of Queensland. The release for the mortgage was signed in September 1889. The house became the family's holiday home. Besides his political endeavours, Gannon was actively involved in Brisbane's social and sporting society. In 1888, Gannon was Vice-President of the Albert Cricket Club, Queensland GovernorAnthony Musgrave was President; he was involved with the Queensland Rifle Association; the Brisbane Bicycling Club; the Breakfast Creek Rowing Club; a Director of the Federal Building, Land Investors Society Pty Ltd and Deposit Bank and a Justice of the Peace. During the early 1890s Gannon's speculative ventures began to fail. The Land Bank of Queensland was in possession of the land by the early 1890s and sold off smaller allotments from the block. Gannon was eventually declared bankrupt in 1895 with liabilities in excess of £97,000. Michael Gannon died in Brisbane on 9 April 1898 aged 50 years after a protracted illness that had forced him to retire. He was buried in the Toowong Cemetery on 11 April 1898. His obituaries described him as "universally respected for his outspokenness and the honourable motives which actuated his conduct" and that he would be remembered as "one of the most honest figures in mercantile circles;... a man who in turn was most happy when sharing his bounty with others less prosperous. Everybody respected him, everybody trusted him. In politics, too, he was known as the 'straight man' whose first consideration was others...".
