He invented a new process, "macadamisation", for building roads with a smooth hard surface, using controlled materials of mixed particle size and predetermined structure, that would be more durable and less muddy than soil-based tracks.

[3] He moved to New York in 1770 and, as a merchant and prize agent during the American Revolution, made his fortune working at his uncle William McAdam's counting house.

[7] Although McAdam was paid £5,000 for his Bristol Turnpike Trust work and made "Surveyor-General of Metropolitan Roads" in 1820, professional jealousy cut a £5,000 grant for expenses from the Parliament of the United Kingdom to £2,000 in 1827.

[citation needed] McAdam died in Moffat, Dumfriesshire, while returning to his home in Hoddesdon, Hertfordshire, from his annual summer visit to Scotland.

His second surviving son, James Nicoll McAdam, the "Colossus of Roads", was knighted for managing turnpike trusts: a knighthood, it is said, previously offered to his father but declined.