Cox scored 12 goals in 17 games in the 1897-98 season. His form was so impressive that he was transferred to Liverpool in the First Division in February 1898. Despite the loss of the club's top scorer, George Allan, Liverpool finished in second place in the First Division that season.
According to Tony Matthews (Who's Who of Liverpool) "He (Cox) was fast and able to centre in splendid style." He did seem to have a major weakness. As the Athletic News pointed out: "Fast, but inclined to indulge in trickery against a weak opponent."
Liverpool was relegated in the 1903-1904 season. However, the club won the Second Division championship in the 1904-1905 season. Cox played an important role in Liverpool winning the First Division title in the 1905-1906 season, when they beat runners-up, Preston North End, by four points.
Jack Cox rejoined Blackpool as player-manager in August 1909. While at Liverpool he scored 80 goals in 360 games. However, he was unable to get Blackpool promoted to the First Division and in May 1911 he retired from the game.
Jack Cox died in Blackpool in 1946.