By 1794 John was a major in an artillery corps. He was proud of his commission, which was one of the last signed by King George III. In 1798 he received a government appointment at the beginning of the Napoleonic War as agent for the revictualisation of the Navy in the western ports. This appointment took him to Falmouth, England. In 1801 his acceptance number in indonesia of the position of surveyor to the Bristol turnpike trustees set in motion the great work of his life. He was then 45 years of age. By 1818 he was acting as consulting surveyor to no less than 34 Turnpike Trusts.
His 3 sons one by one gave up their own careers in Scotland, in each case at some financial sacrifice and moved to England to assist their father. John's work was held in such high esteem that a Parliamentary Committee, reporting in 1819, was distinguished in its praise of him and the efficiency and economy of his method of road maintenance. By the year 1823 the number of Road Trusts consulting McAdam had increased to nearly seventy. Thirty-four of which were managed by the McAdam family. Quinton McAdam of Craigengillan, Quinton McAdam of Waterside and Alexander McAdam of Grimmet, all cousins of John Loudon are found on the Trustee's list.