Born in Churachandpur, Manipur, Thangmuansang started playing football at the age of five. In August 2014, he joined the Pune F.C. Academy (soon turned into the Pune City Academy). After spending time with the youth academy, Thangmuansang was loaned to Chennai City of the I-League.
Thangmuansang made his competitive senior debut for the club on 4 December 2017 against Gokulam Kerala. He started and played the whole 90 minutes as Chennai City drew the match 1–1.
On 17 June 2018, it was announced that Thangmuansang signed for NEROCA in the I-League. He made his debut for the club on 7 November 2018 in a league match against Aizawl. He started the match and earned a yellow card in the 88th minute as NEROCA drew 0–0.
On 14 May 2021, Sebastian joined Indian Super League club Odisha FC on a two-year contract.
- As of 27 March 2021