Born in Linktown of Abbotshall, now part of Kirkcaldy, Fife, he was the eldest son of architect and entrepreneur William Adam and his wife Mary Robertson (1698–1761).
John attended Dalkeith Grammar School, outside the city, although he did not proceed to university as he was already being involved in the family businesses.
Upon William's death in June 1748, John took over the family businesses, which besides designing and building houses, included interests in quarrying, mining and other industries.
Additionally, John inherited his father's position as Master Mason to the Board of Ordnance in North Britain.
[2] John took his younger brother Robert into partnership, and the two profited greatly from the lucrative Board of Ordnance contracts which were had been initiated following the Jacobite rising of 1745.
John continued to look after the family's Scottish interests, investing in further quarrying ventures, and the Carron Iron Works.
He supplied his London-based brothers with capital from the Blair Adam estate, at least until the collapse of a stock market venture by his friend Andrew Fairholme in 1764 left him out of pocket.
[4] Further losses occurred after the failure of Robert's speculative Adelphi development in 1772, and John was forced to mortgage Blair Adam.