He began his career with his hometown side Bedlington United before moving to Scotland where he made his professional debut with Heart of Midlothian in 1910 at the age of 18.
Prior to World War I, he helped the side win their first Welsh Cup in 1912 and the Second Division of the Southern Football League a year later.
He remained a mainstay in the first team for seven more years in the First Division, helping the side finish as runners-up in the 1923–24 season and reach two FA Cup finals, in 1925 and 1927.
[3] He joined Stockport County in early 1911 on a free transfer after being spotted by Fred Stewart playing for Hearts' reserve team.
[4] He made one senior appearance for the side, in a 4–0 defeat to Leeds City in the Football League Second Division on 1 April,[5][6] but Stockport were unable to meet the £200 asking price to keep Hardy on for the 1911–12 season.
[4] In 1911, Stewart left his role with Stockport and took up the managerial position at Southern Football League Second Division side Cardiff City.
He also helped the club to victory in the 1912 Welsh Cup final by defeating Pontypridd 3–0 in a replay to become the first team from the south of Wales to win the competition.
[19] Hardy was ever present for the second time in his career having played in all 42 league matches as he helped the side finish as runners-up and win promotion to the First Division.
[17] Hardy featured in Cardiff's first four matches in the top tier, but his start to the campaign was disrupted by a bout of influenza that restricted him to only two appearances between mid-September and the end of November.
[20] After his first season in the First Division, the Daily Herald wrote "It is impossible to persuade any soccer follower in South Wales that there is a better left-half in the country".
[23] Two months into the following season, Hardy was approached to stand as a candidate for the Labour Party in an election for Cardiff City Council.
[27] In the 1924–25 campaign, Hardy played in all eight matches of the club's FA Cup run as they reached the final before losing 1–0 to Sheffield United.
[28] Two years later, the club reached their second FA Cup final, this time emerging victorious after defeating Arsenal 1–0 to become the only team from outside England to win the competition.
[8] In its match report of the final, the Westminster Gazette stated that Hardy's "tactical knowledge of handling a wing was never seen to greater advantage".
He also helped the club to victory in the Welsh Cup, defeating Rhyl in the final,[30] and the 1927 FA Charity Shield early the following season.
Ahead of Cardiff's 1927 FA Cup victory, the Daily Herald remarked "there is no more popular player in South Wales than "Billy" Hardy".
[44] Hardy retired from playing upon his release and left Cardiff after 21 years to take up a coaching position with Second Division side Bradford Park Avenue.
[48] Hardy gained a reputation as a tenacious half back, a 1922 article in John Bull described him as "a full-blooded 90 minutes player, one of the most relentless tacklers in the game".