He made his first fortune in the USA, after moving to New York aged 14 in 1770 and working as a merchant for his uncle William. He returned to Scotland in 1783, having chosen the wrong side to support in the Revolutionary War, where he bought an estate near Maybole in Ayrshire.
An astute businessman he got involved with the nearby Kaims Colliery supplying coal to the British Tar Company, the local ironworks at Murikirk and became a trustee of the Ayrshire Turnpike, leading to his interest in road construction.
McAdam became convinced of the need to create roads above the surrounding ground using a system of layered rocks, crushed stone and gravel. He deliberately built each one as level as possible, with a slight camber to allow rain to run off into ditches on either side rather than let the water penetrate and damage the foundations. It proved to be the biggest advance in road construction since Roman times and quickly became known as macadamisation.