Born in Sintra, Lisbon District of Angolan descent,[3] Mendes started playing football aged 9,[4] and his idols growing up were Cristiano Ronaldo, Lionel Messi, David Alaba and Marcelo, the two last players being the ones he tried to emulate.

[7] He spent the next four years making a two-hour round trip in order to train every night with his teacher, before moving to Sporting's academy at the age of 14.

[4] He made his professional debut in a 1–0 Primeira Liga home win against Paços de Ferreira on 12 June 2020, as a 72nd-minute substitute for Marcos Acuña.

[38] Mendes earned his first cap for Portugal at under-21 level shortly after having made his club debut, in a 4–0 win in Cyprus for the 2021 UEFA European Championship qualifiers on 4 September 2020.

[5] Six months later, Mendes was called up to the senior team by manager Fernando Santos for 2022 FIFA World Cup qualifying matches against Azerbaijan, Luxembourg and Serbia,[41][42] and subsequently removed from the under-21 squad due to appear in the finals.

[50] During qualifying for Euro 2024, Mendes made one appearance against Luxembourg, assisting Ronaldo's opening goal of the 6–0 win in the ninth minute;[51] he missed the remainder of the campaign with a hamstring injury,[52] as Portugal won all ten matches in Group J.

[58] Known mainly for his offensive capabilities, speed, and technical qualities, Mendes plays mainly as a left-sided attacking full-back or wing-back, which allows him to create danger either from a standing start or whilst on the counter-attack.

Apart from his dribbling, he boasts an excellent crossing ability whether tightly marked or open in space, and his physical prowess allows him to beat out opponents to the ball either with his speed or strength.

[59] Despite his ability going forward, however, the defensive side of Mendes's game has been brought into question by pundits on occasion, who have cited the inconsistency on his decision-making in front of goal, positioning and awareness as areas of weakness in need of improvement.