Scottish-born left-back Dunlop first appeared in Liverpool's first team towards the end of the 1894/95 season and stayed at the club for no less than 15 years! Dunlop was a regular in the following season but was out of action with an ankle injury from late October to the end of February, but despite his absence Liverpool were promoted back to the First Division. Tom Wilkie and Archie Goldie, the latter incidentally born in the same suburb of Kilmarnock as Dunlop, proved to be a sound full-back pair, but Dunlop finally reclaimed his first-team place from Wilkie for the last seven matches of the 1896/97 season and became a regular in the side for the next decade and a prominent member of the squad who won Liverpool's first-ever first division title in 1901, a feat repeated five years later. In July 1911 he was appointed assistant trainer at Sunderland. Dunlop was the team's captain from 1899-1900.

According to Liverpool's match programme from 17 September 1904: "Dunlop plays a characteristic full-back game, and the familiar cry of on the Anfield ground 'Boot it, Dunlop' is fairly suggestive of his method of defence. There can be no two opinions of his ability to kick the ball from almost any position. Always keen and watchful on the field, he betrays his over-anxiety to repel the invader by his terrific lunges, and on his day, there is no more brilliant player than this same Dunlop. A better servant no club ever possessed, and though we may occasionally differ from him as to the methods he employs on the field, on one point we must all agree, that for downright single-mindedness of purpose Dunlop's tactics have never been questioned."