Born in
Pamplona,
Navarre, Williams joined the
youth academy of
Athletic Bilbao in 2013 from hometown side
CA Osasuna. He began his senior career with the club's
farm team,
Basconia, during the
2019–20 season. On 11 May 2020, Williams was promoted to
the reserve team in the
Segunda División B. He made his first-team—and
La Liga—debut on 28 April the following year, coming on as a second-half
substitute for
Jon Morcillo in a 2–2 home draw against
Real Valladolid; his brother Iñaki also came off the bench ten minutes later. It was the first time two siblings had been on the pitch at the same time for the club since
Julio and
Patxi Salinas in 1986. Williams scored his first two goals for the
Lions on 6 January 2022, in a 2–0 win over
Atlético Mancha Real in
the campaign's
Copa del Rey. Seven days later, he scored the winning goal in a 2–1 victory over
Atlético Madrid in the semi-final of the
2022 Supercopa de España. On 20 January 2022, Williams was given a first team contract after meeting a number of clauses. Williams was a starter in four of the first five matches of the
2022–23 La Liga campaign, and was named player of the match in a 4–1 away win against
Elche CF on 11 September 2022. In the first half-hour of the match he pressurised an opponent into conceding an own goal, was tripped to give his side a penalty (converted by
Oihan Sancet) then scored himself via a dribble from the right wing and powerful shot for his first goal in the competition. Coincidentally this was the same venue and opponent where his brother first found the net in the league seven years earlier. He scored again the following week, the decisive goal of a 3–2 victory over
Rayo Vallecano and his first at
San Mamés. After missing two good scoring chances in the
2022–23 Copa del Rey semi-final as Athletic were knocked out by rivals Osasuna, Williams was forced to deactivate his social media accounts temporarily due to the volume and nature of abusive messages sent to him; he recovered from this disappointment to score the winner against
Espanyol in the league four days later. Athletic made a strong start to the
2023–24 La Liga season with Williams providing numerous scoring chances, mainly from the left wing as coach
Ernesto Valverde settled on a front line featuring Iñaki on the opposite flank and
Gorka Guruzeta in the centre, supported by Oihan Sancet. With Williams's contract expiring in 2024 and media speculation linking him with clubs such as
Aston Villa,
Barcelona,
Liverpool and
Real Madrid, his representatives negotiated with the club for several months before a new contract running to 2027 was agreed on 1 December 2023. The final ended in a 1–1 draw and Athletic went on to win a
penalty shootout to claim their first major trophy in 40 years. The club also finished fifth in the league, their highest position for eight years. He was nominated for the
2024 edition of the
Ballon d'Or, finishing 15th. Despite the new contract, its relatively low
buyout clause of approximately €58 million combined with his good form for club and country meant that he continued to be linked with other top clubs, most prominently Barcelona due to his friendship with
Lamine Yamal, throughout the 2024 summer transfer window, though no party paid the release fee and Athletic were not interested in selling; Williams ultimately confirmed that he would be staying for at least one year longer. In the
2024–25 season, Williams's statistical return was similar to the previous campaign, although his performances were periodically affected by injury issues and he was assessed to have "not had a particularly standout season", with the club's strong form in La Liga (in which they qualified for the
UEFA Champions League for the first time in a decade) largely due to a strong defence, while Oihan Sancet was the most effective attacking component. Williams made his first appearances in continental competition, scoring five times including decisive goals in knockout ties against
Roma and
Rangers and being named in the team of the tournament as Athletic progressed to the semi-finals of the
UEFA Europa League, but they were unsuccessful in their efforts to reach
the final (to be staged at San Mamés) and also made early exits from the
Supercopa de España and their defence of the Copa del Rey. In June 2025, speculation over his future intensified, with Barcelona widely reported as having agreed contractual terms with the player but seeking to pay the transfer fee in instalments due to ongoing financial constraints – which also had the potential to complicate his registration upon completion of the move – as Athletic stood firm over their demand for the contractual release clause fee to be paid up entirely, their position entrenched due to the
historic rivalry between the clubs and the Bilbao hierarchy's ongoing irritation towards their counterparts in Catalonia at the manner of the protracted pursuit. and the defacement of a mural of Williams in
Barakaldo by local fans angry at his reported departure, an action repeated after the club condemned the vandalism and paid for a restoration. On 4 July, Williams renewed his contract with Athletic Bilbao in a 10-year deal, set to run until 2035 and with a reported release clause fee of around €90 million; the announcement video included the Barakaldo mural being spraypainted at night with the slogan 'WIN 2035' by a hooded figure, revealed to be the player. The renewal video later won an award for best social media content of the year in Spanish football, but on the field the
2025–26 season was one of frustration for the club as they were weakened by several injuries and struggled to cope with the demands of domestic competitions and the Champions League. Williams was hampered by a persistent
pubalgia issue which caused him to miss several matches (making only three European appearances) and have little impact on others as efforts were made to manage the condition gradually, with surgery ruled out due to the potential for further complications. Williams occasionally showed flashes of his best form and scored a solo effort against
Real Oviedo in November 2025 which was voted as Goal of the Month, but by February 2026 it was clear that the situation was of little benefit to the player or team and he was withdrawn from the Athletic squad to undergo more intense treatment. ==International career==