In 2017–18, his second season, Sessegnon helped Fulham achieve promotion to the Premier League via the play-offs, scoring fifteen goals and winning numerous personal awards.

'[11] Sessegnon attended Coombe Boys' School in New Malden, London, close to Fulham's Motspur Park training ground.

[12] With both brothers in the team, Coombe Boys won youth competition the ESFA PlayStation Schools' Cup in both 2014 and 2015,[13] Ryan scoring twice in the 2015 final.

[15] Having started training with the first team, still aged 15, in March 2016, Sessegnon was promoted to the senior squad by manager of the year Slavisa Jokanovic for the 2016–17 pre-season.

[16] On 9 August, Sessegnon made his first-team debut in an EFL Cup match against Leyton Orient aged 16 years and 81 days.

[27] Fulham striker Neeskens Kebano called him "the future of English football",[28] with teammate Stefan Johansen saying it was "99% certain he will be one of the top left-backs in the world".

[29] As the summer transfer window approached, interest was reported from numerous Premier League clubs, including Everton,[30] Liverpool[31] and Manchester United,[32] with Sessegnon potentially available for a compensation fee of £2 million[33] as Football Association rules meant he could not sign professional terms until his 17th birthday on 18 May 2017.

[41] After netting six goals in January 2018, courtesy of braces against Ipswich Town, Burton Albion and Barnsley, Sessegnon was named PFA[42] and Championship Player of the Month.

[47] On 26 May, Sessegnon assisted Tom Cairney's 23rd-minute goal in a 1–0 win against Aston Villa in the Championship play-off final at Wembley to secure Fulham's promotion to the Premier League.

[54] His Champions League debut came in the following match, as he came off of the bench against Red Star Belgrade and assisted a goal for Christian Eriksen.

[66] He went on to score three goals, including two in a 4–1 win over Germany,[67] finishing the tournament as joint-top scorer[68] as England were crowned champions for the first time.

[70] Aged 17, Sessegnon made his debut for England Under-21s on 27 March 2018, starting in a 2–1 European Under-21 Championship qualifier win over Ukraine.

[72] On 27 May 2019, Sessegnon achieved the Guinness World Records title for 'Fastest time to hit both football goal posts and crossbar', doing so in just 7.75 seconds.