Seny was born in Zurich to a Senegalese father and Swiss mother.

Completing his education at the Sport Academy Zurich, his youth career was spent with two clubs in the city, Red Star and Grasshoppers.

His senior debut came on loan from Grasshopers at FC Grenchen, while he also later played for German club MSV Duisburg.

After training with a number of clubs in England, he joined Queens Park Rangers in August 2016. 

His first experience of football in this country was on loan with Whitehawk in the National League South. Here he would score a late goal in a match against Chippenham Town, a feat he’d later repeat in the Sky Bet Championship.

His early years of development were spent on loan with a series of clubs also including Hampton & Richmond Borough, Stevenage, Dundee and Doncaster Rovers. 

Seny’s first league start for QPR was at home to Boro, a 1-1 draw in September 2020. Chuba Akpom was Boro's goalscorer that day.

Seny would win QPR’s Player of the Month twice that season, in November and January, and after a total of 42 appearances and 11 clean sheets, he was named Rangers’ Players’ Player of the Year alongside Rob Dickie.

Seny was handed the number 1 jersey at Loftus Road ahead of the 2021/22 campaign. 

He would make a total of 121 appearances for QPR, keeping 30 clean sheets. 

Perhaps his standout moment of the 2022/23 season was when he nodded in a late equaliser, the second goal of his career, away to Sunderland at the Stadium of Light. It was later selected as QPR’s Goal of the Year.

Seny received his first call-up for Senegal in 2014, but had to wait until March 2021 for his national team debut in a 1-1 draw with Eswatini to help score qualification for the 2021 Africa Cup of Nations.

He was named in the Senegal squad for the tournament, and with first choice Edouard Mendy ruled out due to COVID-19, kept a clean sheet in their opening row matches as Les Lions progressed through the group stages. 

Senegal won the Africa Cup of Nations 2021, beating Egypt on penalties in the final, and Seny and his teammates were later appointed Grand Officer of the National Order of the Lion by President of Senegal Macky Sall.