Kyle Vassell began his career as a schoolboy at Norwich City and joined Brentford at age 15 after having difficulty with commuting from his London home to Norfolk.

He failed to feature in the first team while at Brentford and spent work experience loan spells at non-league sides Woking and St Albans City during the 2010/11 season. He was released by Brentford in June 2011 after he completed his two-year scholarship and later felt that he had given away his opportunity at being a professional footballer.

Following interest from Gillingham and Burton Albion and an unsuccessful trial with League Two side Crewe Alexandra, Vassell signed non-contract terms with Conference South side Dover Athletic in October 2011, linking up with former Brentford manager and player Nicky Forster. He scored on his debut against in a 6-3 Kent Senior Cup win over Maidstone United on 12 October. He made three further appearances before leaving the club.

Vassell signed non-contract terms with Conference South side Sutton United on 30 November 2011 and scored on his debut in a 1-0 Surrey Senior Cup win over Epsom & Ewell on 6 December. Vassell signed dual-registration deals with Isthmian Division One South sides Tooting & Mitcham United and Whitehawk respectively and signed for Conference South side Staines Town on loan on 21 March 2012. During the 2011/12 season, he made fifteen Sutton appearances, scored seven goals and helped the club into the Conference South playoffs

Vassell signed for Conference South side Chelmsford City on 6 December 2012. He helped secure a fifth-place finish for Chelmsford, but he suffered playoff heartbreak for the second season running as Chelmsford were beaten in the semi-finals by the eventual promoted team Salisbury City. He finished the season having made 25 appearances and scored 6 goals

Vassell signed for Chelmsford City's neighbours and divisional rivals Bishop's Stortford on 30 June 2013. He scored a hattrick in his first competitive game for the club, a 6-1 Herts Charity Cup win over Ware on 1 August. 16 goals in 16 games led to considerable interest and he signed two-and-a-half-year deal with League One side Peterborough United for an undisclosed fee on 11 November 2013. Vassell made his Peterborough debut (his first appearance in the Football League) on 24 November, starting in a 1-0 home league defeat to Stevenage.

He joined Oxford United on loan on 26th March 2015 until the end of the season.

In the 2015/16 season, he made seven appearances for Peterborough, without scoring, before being loaned to Dagenham & Redbridge on 6th November 2015 until 2nd January 2016. He scored three times for Deganham.

On 8th January 2016, he was loaned to Shrewsbury until the end of the 2015/16 season.

Vassell was released by Peterborough on 13th May 2016 and was linked with a move to Crawley. On 16th June 2016, he signed a one year contract with Blackpool.

He moved to Rotherham in the summer of 2018. He made his international debut for Northern Ireland against Austria on 12th October 2018.
He joined Fleetwood on loan on 15th January 2021 having played 63 times for the Millers scoring six goals. Vassell scored four goals in 26 appearances for Fleetwood.

He was released by Rotherham on 17th May 2021. He then joined Cheltenham on 9th August 2021 on a one year deal.

He ended his contract with Cheltenham on 6th January 2022 signing for American side San Diego Loyal on 11th January. They play in the USL Championship.

He signed for Scottish Premiership side Kilmarnock during the January 2023 transfer window.