Dack then joined Gillingham in 2008 and started a three-year scholarship in 2010 with Gillingham, signing his first professional contract in May 2012. He enjoyed a prolific pre-season in 2012 including a four-goal haul against Royal Engineers, which attracted attention from Gillingham manager Martin Allen, leading to his professional debut on 14 August 2012, in a 2–1 victory over Bristol City in the Football League Cup. He made his league debut against Bradford City on 18 August 2012, being narrowly denied a late goal by the upright. He scored his first senior goal on 4 September 2012 in the Football League Trophy away to Crawley Town and signed a new three-year contract the following day. Dack received his first red card as a professional for violent conduct against Southend United on 18 September 2012, having come on as substitute, after first winning a penalty. On 3 January 2013, Dack moved on a one-month loan to Football Conference side Braintree Town. He was named as Gillingham's Young Player of the Season at the end of the 2012–13 season, having made 20 appearances for the club in all competitions, scoring 2 goals. On 16 April 2014, Dack signed a new four-year deal keeping him contracted at Gillingham until June 2018. He was named Gillingham's Young Player of the Year, Sponsor's Player of the Year and Players' Player of the Year at the conclusion of the 2014–15 season, having scored 10 goals in 50 appearances in all competitions. In the 2015–16 season Dack was Gillingham's top goalscorer, with 15 goals in 44 appearances in all competitions and 11 assists in the league. In the January 2016 transfer window he was reportedly the subject of three bids, including one of £2 million, from Championship side Bristol City, but all three were rejected by the club. At the 2015–16 Football League Awards Dack won the League One Player of the Season award and was named in the Football League Team of the Season. The same season he was also named as Gillingham's Player of the Year, Young Player of the Year, Players' Player of the Year and Sponsor's Player of the Year. In the 2016–17 season he was named as Gillingham's Young Player of the Season for the third year running. He finished the season with 6 goals in 39 appearances.
Blackburn Rovers
On 27 June 2017 he signed for League One side Blackburn Rovers for an initial fee of £750,000. He scored his first goal for the club against Shrewsbury Town on 23 September 2017. In his first season for the club he scored 18 goals in 42 league appearances and won the EFL League One Player of the Season award.
Style of play
An attacking midfielder, Dack has described himself as adopting a "free role behind the two strikers in the final third to go and pick up the ball, get into good positions and create and score goals". Dack's manager at Gillingham between 2015–2017, Justin Edinburgh, has described him as "the catalyst of team..full of energy, he is tenacious, he has quality and he is a top player – definitely Championship quality", adding that he "probably covers more ground than any other player". Interviewed by Sky Sports in February 2016, Dack suggested that the model for his own style of play was former Chelsea and England international Frank Lampard. He also revealed that before committing to football he had undertaken trials to play cricket for Kent.
