Hardcastle joined the Academy at Blackburn Rovers at the age of six. He played 12 games for the under-18 team during the 2013–14 season. He appeared for the under-21 team in the second leg of the Under-21 Premier League Cup final defeat to Southampton at St Mary's Stadium on 20 April 2015, coming on as a 57th-minute substitute for David Carson in what ended as a 2–1 defeat after extra-time. He scored two goals in 22 appearances for the under-18's during the 2014–15 season. On 24 September 2015, Hardcastle joined Northern Premier League Premier Division side Salford City on a one-month loan deal. The 17-year old's loan deal was later extended to three months, after which time co-manager Anthony Johnson said that "we’re devastated to be losing him because he’s been an integral part of what we’ve done". He helped the "Ammies" to reach the Second Round of the FA Cup, eliminating Notts County in the First Round with a 2–0 win at Moor Lane. He signed his first professional contract with Rovers whilst at Salford, after manager Paul Lambert gave him a -year contract. He returned to playing for the "Riversiders" youth-team, and scored the only goal of the game against Luton Town in the quarter-finals of the FA Youth Cup. He scored three goals in 26 youth-team appearances during the 2015–16 season and scored two goals in 33 youth-team appearances during the 2016–17 season, including three appearances in the EFL Trophy. In November 2017 he signed a new contract to keep him at Ewood Park until the summer of 2019. He went on to captain the under-23 team to the Premier League 2Division 2 title at the end of the 2017–18 season. On 30 August 2018, Hardcastle joined EFL League Two side Port Vale on loan for the rest of the 2018–19 season. He said that "the gaffer said to me I needed to go out and play games in men’s football, come back strong and fit and you never know" and that he was "excited about the challenge" in playing in the English Football League. He made his first-team debut two days later after starting in a 2–1 defeat to Newport County at Vale Park. However he made only five starts and four substitute appearances for Neil Aspin's "Valiants" before he was recalled to Blackburn on 7 January.
Barrow
On 8 February 2019, he joined National League side Barrow on an initial one-month youth loan. The deal was made into a permanent move on 28 March after he impressed "Bluebirds" boss Ian Evatt with his dynamism and bravery in midfield; he signed a contract to run until summer 2020. He scored three goals in 41 appearances in the 2019–20 season, which was permanently suspended on 26 March due to the COVID-19 pandemic in England, with Barrow top of the table. Though the season was not resumed, Barrow went on to be promoted to the Football League as National League champions.
