[2][3][4] Graduating from Sporting CP youth system, Leão made his first-team debut in 2018, winning the 2017–18 Taça da Liga, before unilaterally terminating his contract, following an incident involving a violent mob of ultras fans within the club's facilities.

[10] Courtesy of manager Jorge Jesus, Leão first appeared in the Primeira Liga with the first team on 11 February 2018, replacing Bryan Ruiz for the last 21 minutes of the 2–0 home win over Feirense.

[11][12] On 2 March, having taken the place of injured Seydou Doumbia late into the first half, he equalised for the visitors a few minutes after taking the pitch but in a 2–1 away loss to Porto[13]– in the process, he became the club's youngest-ever scorer against that opposition.

[19] Sporting CP would later dispute Lille's deal to sign the player, deeming the transfer "unacceptable", filing a complaint with FIFA and claiming the €45 million of his release clause.

[34] On 29 October, he scored his first goal in European competitions, in a 3–0 home group stage victory in the UEFA Europa League against Sparta Prague.

[35] He scored the fastest goal in the history of Serie A and in Europe's top-five leagues, after six seconds on 20 December at Sassuolo; the match ended 2–1 for his team.

[47] Seven days later, Leão's two assists to Sandro Tonali helped his team come from behind to defeat Hellas Verona 3–1 away from home, allowing his side to overtake rivals Inter at the top of the Serie A table.

[48][49] On 22 May, he provided a hat-trick of assists for the first time in his career as Milan beat Sassuolo 3–0 away from home to win their first Serie A title in eleven years.

[59][60] On 18 April, he provided an assist to Olivier Giroud following a solo run, to be later named player of the match,[61] in a 1–1 away draw against the same opponent in the 2022–23 UEFA Champions League quarter-final second leg, which secured his team's progress to the semi-final, by winning 2–1 on aggregate, for the first time since the 2006–07 season.

[74] The shirt had also previously been worn in the Serie A by former Milan players, most notably Clarence Seedorf, Gianni Rivera, Roberto Baggio, Rui Costa, and Ruud Gullit.

[77] Over the following months, Leão was criticized for his performances, most notably in Milan's Champions League group stage match against Newcastle United, which ended in a 0–0 draw.

[78][79] In the Europa League, Leão helped his team reach the quarter finals of the competition, scoring two goals in the knockout round play-offs against Rennes in a 5–3 aggregate win.

[80] In the round of 16, he scored one goal and provided two assists in a 7–3 aggregate win over Slavia Praga, helping Milan reach the quarter finals of the competition.

As Milan struggled for form in Serie A, Fonseca opted to bench Leão for several matches, favoring players like Noah Okafor, who were perceived to offer more balance to the team's structure.

[92] On 6 January 2025 Leão won the 2024 Supercoppa Italiana against rivals Inter Milan, under the new coach, Sergio Conceição (who replaced Paulo Fonseca).

In the finals against Spain, Leão replaced Domingos Quina in the 79th minute; Portugal went on to win the competition for the sixth time, following a 5–4 penalty shoot-out victory after a 1–1 draw in extra-time.

[96] At the 2019 FIFA U-20 World Cup in Poland he played all three games and scored the only goal in a 1–1 draw against South Africa, though his side did not advance from the group.

[97] On 10 November 2017, aged only 18, Leão won his first cap for the Portugal under-21 side, playing 31 minutes in a 1–1 away draw against Romania for the 2019 UEFA European Championship qualifiers.

[102] Four days later, he made his international debut, replacing Cristiano Ronaldo at half-time and providing the assist for André Silva's goal in a 3–0 friendly defeat of Qatar.

[117] Operating primarily on the left-hand side of Stefano Pioli's 4–2–3–1 system, Leão gives Milan a creative spark in wide areas where he particularly is involved in one-on-one situations, with his direct style of attacking, allowing him to aggressively accelerate past the defenders, where he can create big chances and engineer good shooting opportunities for himself, as opposed to dribbling past an opponent.

[17] Tiago Fernandes, who worked with him at the Sporting youth academy, told French newspaper L'Équipe that in his opinion the player was better than Cristiano Ronaldo at the same stage.

[123] Portugal teammate Bruno Fernandes, stated Leão reminds him of Brazilian footballer Ronaldo "with his speed, technique and explosiveness in decisive moments, being qualities that have emerged every game to drag Milan towards great results.”[124] At times, Leão had been criticized for lacking consistency in his performances, unwillingness to participate in the defensive play, as well as unnecessary flamboyance while in possession of the ball; former AC Milan and Italy national team coach Arrigo Sacchi, while acknowledging the player's individual talent, called Leão a liability to the AC Milan team as a collective.