[11] Mawson made his professional debut in a 5–0 League Cup second round defeat at Derby County in August 2013, replacing Sam Saunders after 73 minutes.

[12] He scored his first goal of the 2013–14 season with the opener in a 1–1 Professional Development League 2 South draw with Brighton & Hove Albion on 16 September 2013.

[17] A torn meniscus in a Development Squad match ended Mawson's season early, prompting him to contemplate quitting football,[18] before signing a new one-year deal on 5 June 2014.

[22] Mawson signed for Conference South club Maidenhead United on 13 December 2012 on a one-month loan,[23] earning praise from manager Johnson Hippolyte for his performance on his debut in a 1–0 win against Truro City.

[28] On 28 November 2013, Mawson moved on loan to Conference Premier club Luton Town until 4 January 2014, to cover for injuries to defenders Steve McNulty, Andy Parry, Alex Lacey and Anthony Charles.

[28][29] He made his debut in a 5–0 away league victory over Alfreton Town on 7 December, but was substituted for Joe Davis at half time due to an injury.

[34] A knee injury disrupted Mawson's time at Welling and he returned to Brentford at the end of the season, having made 9 appearances and scored one goal.

[17][35] Mawson joined League Two club Wycombe Wanderers on 9 August 2014 on a one-month youth loan, as cover for the injured Gary Doherty.

[36] He made his Wycombe debut (the first Football League appearance of his career) on the opening day of the 2014–15 season, playing the full 90 minutes of a 2–0 away win over Newport County alongside former Brentford teammate Aaron Pierre at centre back.

[40][41] He returned to action on 29 November against Hartlepool United and capped Wycombe's afternoon with the first Football League goal of his career in a 3–1 victory.

[48] Mawson's 97th-minute winner in a 3–2 victory over Northampton Town on the final day was not enough to prevent Wycombe from having to settle for fourth place and a play-off semi-final matchup versus Plymouth Argyle.

[60] He made his full debut on 22 October 2016 in the 0–0 home draw against Watford[61] in the Premier League after which he presented with the man of the match award for his work in defence.

[65][66] After Swansea's relegation from the Premier League, Mawson signed for newly promoted club Fulham on a four-year contract for an undisclosed fee, reported to be in the region of £20 million.