Deng has four older siblings as well as a half-brother who lives in Uganda.[11] His older brother, Peter Deng, has represented South Sudan at senior level.[12]
Deng made his senior football debut as a 16 year old in 2013 at Western Eagles F.C. under then-manager Budala Barešić-Nikić in the Victorian State League Division 3.[14][15]
The following season, the youngster moved to nearby Green Gully SC, 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.[6]
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.
In June 2016, Deng moved to Jong PSV on a one-year loan deal.[16][17] After making five appearances, he returned to Melbourne Victory at the end of the season.[18]
He 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.[20]
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.[21][22]