McNair was born in Boston, Lincs, but moved with the family to Tyneside, where his father became a brewery drayman. Despite poverty, McNair recalled his childhood with happy nostalgia. He left school at 13, eventually getting work at Wallsend Slipway, but was in trouble with his employers because of his left-wing sympathies. He had earned the title of 'the boy orator of Tyneside', while speaking as 'John Brown'.
He moved to Alfred Herbert's engineering concern in Coventry. Recognising his talent, the firm sent him to manage their Paris operations, though he did not know a word of French. He stayed in France for 25 years, and was within earshot when Jean Jaures, the French socialist, was assassinated in 1914.
McNair returned to England in 1923, but exhausted himself in working for the Independent Labour Party, and returned to France on medical advice. There he became a leather merchant and founded a football club with eight teams (in 1955, the sports ground at Le Vestinet outside Paris, was named, at de Gaulle's suggestion, Stade McNair). This all-round man also lectured on English poets at the Sorbonne. He was the first British worker to go to Spain, where he remained as the I.L.P. representative in Barcelona from August 1936 until June 1937. There he encountered George Orwell, who fought with the P.O.U.M. on the Aragon front and describes his experiences in the celebrated book Homage to Catalonia. McNair left a manuscript 'George Orwell: The Man I knew', dated March 1965, based on his MA thesis. One copy is in the University of Newcastle Library. In February 1937, Orwell's wife Eileen (born in South Shields) arrived in Barcelona, to be nearer her husband and to work as McNair's secretary,
McNair was appointed general secretary of the I.L.P. in 1939 and served in that capacity until 1965. He was a frequent contributor to the New Leader, the I.L.P. weekly (later the Socialist Leader) and wrote the official biography of James Maxton, the leader of the I.L.P., entitled The Beloved Rebel (1955). Maxton and William Morris were his heroes and he himself exemplified 'socialism with a human face'. Late in life McNair enrolled at Durham University and graduated just before his 71st birthday. A few years later he gained his Master's degree. He died at North Shields at the age of 80.