Sion Fernandes is a youth exponent from
FC Sion. He made his
Swiss Super League debut on 1 June 2013 against
FC Zürich in 4–2 home win. He started in the first eleven and was substituted after 53 minutes. He scored his first senior goal on 1 March 2015 in a 2–2 draw at home to
FC Luzern. He made his West Ham debut on 25 September 2016 in a 3–0 home defeat by
Southampton, coming on as an 82nd-minute substitute for
Mark Noble. On 26 October 2016, Fernandes scored his first goal for West Ham in a 2–1 win against
Chelsea in the
EFL Cup, winning praise from manager
Slaven Bilić.
Loan to Fiorentina On 13 August 2018, Fernandes joined Italian side
Fiorentina on a one-year loan deal with an option to buy.
Mainz 05 On 3 June 2019, Fernandes joined
Bundesliga side
Mainz 05 on a four-year deal for an undisclosed fee. On 17 August 2019, he made his full debut for the club in the league season opener against
SC Freiburg, starting and playing 84 minutes of a 3–0 loss while picking up a first-half yellow card. Fernandes scored his first goal for Mainz in the penultimate
Bundesliga game of the
2019–2020 season. After being substituted on in the second half, he scored the third goal in Mainz's 3–1 win over
Werder Bremen.
Loan to Young Boys On 15 February 2022, Fernandes moved on loan to
Young Boys until the end of the season.
Loan to Brest On 30 August 2024, Fernandes signed for
Ligue 1 club
Brest on loan for the season, with an option for the club to make the deal permanent.
Return to Young Boys On 14 July 2025, Fernandes returned to
Young Boys on a permanent basis and signed a four-year contract with the club. ==International career==