His two elder brothers had died earlier as small boys, so he was effectively the eldest son of English-born emmigrants, George and Ann Mawbey, who had married in Sydney in 1838.
At the time of his birth, his father had just started working as the schoolmaster at the Church of England diocesan school at Dural.
John Thomas was baptised in the local Church of England, St Jude's.
At the end of 1860, when he was 11, the family moved back to Sydney, to the inner suburb of Newtown.
Two years later, when he was 13, his father died of a stroke leaving his mother with eight children between the ages of 17 years and three months to raise on her own.
When old enough to work, John Thomas was a carrier, transporting goods with a horse and cart, as was his younger brother, George.
He contributed some of the money he made to the fund to build St Stephen's Church of England at Camperdown.
When John Mawbey married Sarah Clarke in Mudgee in 1875, his occupation was a 'dealer', in other words, a businessman, someone who buys and sells goods.
He moved to Mudgee and ran a fruit shop there for about six years before taking up a selection of land at Breelong in 1884 when the old squatters' holdings were carved up for smaller farmers.