[3] While progressing in the youth at Lesquin, he almost joined Leeds United in September 2008, but both parties could not agree terms after Lens intervened in the transfer and demanded a compensatory fee.

[4] At some point on progressing his youth career, Knockaert thought about considering quitting football after being told by the Lens' management that he was released because he was small at the time.

[9] Seven days later, on 29 September 2010, he made his league debut, making his first start and played 69 minutes before being substituted, in a 5–0 win over Gap.

[10] Soon after, he signed a three-year contract with the club on 6 October 2010[11] Knockaert then scored his first league goals, in a 6–0 win over Maccabi Paris Métropole on 22 April 2011.

[15] In the ten matches between 19 December 2011 and 16 March 2012, he scored six times against Sedan,[16] Istres,[17] Boulogne,[18] Le Havre,[19] Troyes[20] and Lens.

[28] Knockaert made his Leicester City debut on 18 August 2012, where he came on as a substitute for Lloyd Dyer in the second half, in a 2–0 win over Peterborough United.

[38] However, in the 2013 Playoff semi-final second leg, with the aggregate scores level he allegedly dived to win a penalty for Leicester City deep into stoppage time.

In the 2013–14 season, Knockaert scored his first goal of the 2013–14 season in the 5–2 League Cup victory away to Carlisle United[42] and scored again in the second round of the League Cup with a 30-yard free kick at home to Derby County and then assisted Danny Drinkwater's winning goal in a 2nd half come back in the 3rd round victory.

[49] However, he struggled to regain his first team place, owing to the performances of Nick Powell, Marc Albrighton and Danny Simpson.

[58] Knockaert started well on his Standard Liège debut when he scored his first goal for the club despite losing 2–1 to Kortrijk in the opening game of the season.

[59] Five days later, he scored again, but this time in an UEFA Europa League match against Željezničar Sarajevo, as well as setting up a goal in a 2–1 win.

[67] Between July 2015 and January 2016, Knockaert made 20 appearances and scored 5 goals in 2015–16 for the club despite missing out a game, due to suspension after picking a yellow card for the fifth time this season against Anderlecht on 8 November 2015.

[68][69] On 6 January 2016, Standard confirmed they had accepted an offer from Championship side Brighton & Hove Albion with the player set to travel to England to agree personal terms.

[72] Knockaert made his Brighton debut five days after signing with the club on 12 January 2016, playing the full 90 minutes in a 2–0 defeat against Rotherham United.

[75] Knockaert was a key performer for Brighton in April 2016, scoring against Burnley,[76] Fulham[77] and twice against Queens Park Rangers, as the club pushed for automatic promotion.

[79] Despite Brighton's unsuccessful attempt to reach promotion to the Premier League after losing in the play-offs to Sheffield Wednesday, Knockaert impressed for the club, making 16 appearances and scoring 5 times.

In the 2016–17 season, Knockaert was linked with a move to recently relegated side Newcastle United, but Brighton rejected their bid.

[96] On 9 March Knockaert scored a screamer which turned out to be the winner against bitter rivals Crystal Palace where it finished 2–1 at Selhurst Park.

[99] On 21 July 2019, Knockaert joined Fulham on a season-long loan from Brighton & Hove Albion with an option to make the move permanent.

On 16 October 2020, Knockaert joined Championship side Nottingham Forest on a loan until 7 January 2021 in where he was to play under former Brighton manager Chris Hughton.

[113] Knockaert joined French Ligue 2 club Valenciennes in September 2023 after Fulham confirmed that his contract was cancelled by mutual agreement.