On 19 September 2011 Seityasen made his professional debut in the Federation Cup against Churchill Brothers for Royal Wahingdoh who were then in the I-League 2nd Division.

[6] He would then make his I-League debut against Shillong Lajong and scored his first goal of the 2014-15 season from a Jackichand Singh assist in a match they won by 2-1 scoreline.

[10] On 24 January, Seityasen made his league debut, coming-on as 38th-minute substitute for Alesh Sawant in an away match against Mumbai FC at the Cooperage Stadium.

On 23 July 2017, Delhi Dynamos FC confirmed that they have signed NorthEast United's star winger Seityasen Singh.

[12] He played his first match of the 2021–22 Indian Super League season against defending champions Mumbai City FC on 19 November 2021 as a substitute for Álvaro Vázquez in the 83rd minute of the game which they won 0–3 at full-time.

[14][15] On 11 June 2015, Singh made his debut for the India national football team in the 2018 FIFA World Cup qualifier against Oman.