Deng has four older siblings as well as a half-brother who lives in Uganda.[9] His older brother, Peter Deng, has represented South Sudan at senior level.[10]
The following season, the youngster moved to nearby Green Gully, where manager Bob Stojcevski recruited Deng to play for the club's u20 side. He was eventually promoted to the senior team and made 13 first team appearances in the NPL Victoria in 2014.[4]
Melbourne Victory
Following his impressive performances at Gully, Melbourne Victory National Youth League coach Darren Davies rewarded Deng with a youth contract. Deng debuted for the senior team for Melbourne Victory against Balmain Tigers in the 2015 FFA Cup. He made his A-League debut on 9 October 2015 against Adelaide United, as a substitute replacing Fahid Ben Khalfallah after a red card was shown to Leigh Broxham. Thomas made his starting debut in the A-League for Melbourne Victory on 17 October 2015 in round 2 against town rivals, Melbourne City, where he was selected as Man of the Match by supporters and social media.
Jong PSV (loan)
In June 2016, Deng moved to Jong PSV on a one-year loan deal.[14][15] After making five appearances, he returned to Melbourne Victory at the end of the season.[16]
Urawa Red Diamonds
On 28 January 2020, Deng left Melbourne Victory to join Japanese club Urawa Red Diamonds following his wonderful performance at the 2020 AFC U-23 Championship.[17] On 23 August in a league match against Vissel Kobe, he scored a goal in a 2–1 lost.
Albirex Niigata
On 9 January 2022, Deng moved to J2 League club Albirex Niigata. In his first season at the club, he helped them to win the J2 League title thus gaining promotion to the top tier J1 League in 2023. On 6 January 2024, Deng's contract was extended for the 2024 season.[18]
Deng was part of the main squad of the Olyroos at the 2016 AFC U-23 Championship and put on an extremely impressive display throughout the tournament, garnering high praise from respected Australian pundits: Geoff Fullgrabe, Alan Vucenik, and Craig Foster.
In October 2018, Deng was named in the Socceroos squad for their training camp in the UAE and their friendly against Kuwait. In the friendly he made his debut, together with fellow South Sudanese refugee and boyhood friend Awer Mabil.[19]
On 22 July 2021, Deng captained Australia in a historic 2–0 win over Argentina in the first group game of the 2020 Summer Olympics in Tokyo, also winning the Player of the Match award.[20][21]
Deng was included in the Australian squad for the 2022 FIFA World Cup and the 2023 AFC Asian Cup but did not play a single match at both tournament.