He was born in Ayr, Scotland. He moved to New York in 1770 as a young man and made his fortune working at his uncle's counting-house with a successful career as a merchant and prize agent during the American Revolution.
He returned to Scotland in 1783 and purchased an estate at Sauchrie, Ayrshire. As an estate-owner and road trustee, he then commenced work on finding ways to improve the notoriously bad roads of Great Britain. His eventual conclusion was that roads needed to be raised above the surrounding ground, and carefully constructed from layered rocks and gravel. He wrote two treatises documenting his research, Remarks on the Present System of Road-Making (1816) Practical Essay on the Scientific Repair and Preservation of Roads (1819). In 1820 Parliament awarded him 2,000 pounds for his efforts and in 1827 he was made Surveyor-General of metropolitan roads.
When he was appointed surveyor to the Bristol Turnpike Trust in 1816 he remade the roads under his control with crushed stone bound with gravel on a firm base of large stones. A camber, making the road slightly convex, ensured the rainwater rapidly drained off the road and did not penetrate the foundations. This way of building roads later became known as the Macadamized system.
When a macadam road was built, side ditches were dug, and the road bed was laid with three layers of decreasingly-sized rocks, carefully pulverized "so as not to exceed 6 ounces in weight or to pass a two-inch ring." The finished road was compacted with a cast-iron roller, and the compaction process was completed by passing traffic.
As a result of his success, McAdam was made surveyor-general of metropolitan roads in England. By the end of the 19th century, most of the main roads in Europe were built in this way. John McAdam died at Moffat, Scotland.
The first macadamized road in North America was completed in 1830.
Although macadamization was replaced by more modern techniques in the early 1900s, the name lives on. Tarmac was originally marketed as tar-macadam, because it was a macadamized road incorporating a binder of tar.
Modern road surfaces are still largely dependent on McAdam's discovery. Coal tar was first used to bind the stones together, hot-laid tarred aggregate or tar-sprayed chips providing an excellent road-metalling for the surface. Oil-based asphalt from Trinidad and Tobago and from refineries was later used as a road surfacing, laid on reinforced concrete, but still owes a lot to McAdam as it is mixed with granite or limestone chippings. This process became known as Tarmacadam (a short form of which is used to refer to airplane runways: "tarmac").
McAdam never really achieved the respect that was his due. He was paid the sum of £5,000 for works done for Turnpike Trusts around Bristol, but a proposal for £5,000 from Parliament as a grant for his expenses was first refused, then cut to £2,000, mainly due to professional jealousy. Corruption in roadworks was appalling; by his own efficiency, MacAdam exposed the abuse of road tolls by less scrupulous Turnpike Trusts, many of which were run at a deliberate loss despite high toll receipts. Travellers of all kinds respected McAdam, but those whose scams he had revealed remained his bitter enemies. His reputation has nevertheless survived, as the Scotsman who paved the way for development.