Nico Williams was born in Pamplona, Spain, to Ghanaian parents who had sought refuge in the country, and grew up in the Athletic Bilbao system after joining their academy in 2013 as an eleven-year-old. He worked his way through the club's reserve structure and made his La Liga debut in April 2021, coming off the bench alongside his elder brother Iñaki in the same match — the first time two siblings had appeared together for Athletic since 1986.
A quick, direct winger comfortable on either flank, Williams made his mark in the 2022–23 season with a string of decisive contributions in La Liga, and went on to win the Copa del Rey with Athletic in 2024, providing the assist for the only goal in a final that went to penalties against Mallorca.
On the international stage, Williams made his senior debut for Spain in September 2022 and was part of their 2022 FIFA World Cup squad. At UEFA Euro 2024 he was one of Spain's most consistent performers, scoring in the round of 16 against Georgia and then opening the scoring in the final against England — a match Spain won 2–1 to claim the trophy for a record fourth time. He was named Player of the Match in the final.
Athletic Bilbao remain his club, where he signed a long-term contract extension through 2035 in July 2024 after resisting significant transfer interest. He has been a regular starter under Ernesto Valverde and contributed five goals in Athletic's run to the UEFA Europa League semi-finals in 2024–25.