Sy is a youth exponent from AS Monaco. He made his first team debut on 20 May 2017 in the final Ligue 1 match of Monaco's championship season against Rennes, a 3–2 away win; he started the match and kept a clean sheet before being replaced by fellow debutant Loïc Badiashile at halftime.[2] He and fellow goalkeepers Danijel Subašić and Diego Benaglio were all released in June 2020.[3]
Nacional
On 11 February 2021, Sy signed a short-term contract with Portuguese Primeira Liga strugglers Nacional.[4] On 24 May, after not appearing officially for the club, it was announced that his contract had been terminated, following the Madeira side's relegation to the second tier.[5]