He became a prolific goalscorer in his 15 seasons at the club, winning the Pichichi Trophy as the highest scorer in La Liga on six occasions.

During his career, Zarra scored a total of 251 league goals, a Spanish record that lasted nearly six decades before being broken by Lionel Messi.

Zarra signed his first professional contract with SD Erandio Club of the Second Division, and was incorporated into team during the 1937–38 season.

Around this time, Athletic Bilbao was looking for players to rebuild the team, which had been disbanded years before in the chaos caused by the Civil War.

Upon his return to Bilbao, Zarra experienced one of the biggest disappointments of his career while playing in the final of the Copa del Rey against Barcelona.

During extra time, Zarra had an opportunity to score against rival goalkeeper Miró, but he missed the shot; consequently, Barça won the match and Cup.

There they met Real Madrid and this time, Zarra was able to redeem himself by scoring the only goal in the final, winning the year's cup in addition to the league title.

Once recovered, Zarra was able to return to the pitch and celebrate with a Copa del Rey title, helping Athletic record a 2–0 win over Valencia.

On 28 May 1950, Zarra again played in another Copa del Rey final, this time against Real Valladolid after Valencia CF was disbanded.

Once qualified, the Spanish team played matches against the United States, Chile, and England in the 1950 FIFA World Cup.

Zarra's goal past the English goalkeeper Bert Williams won the match and sent Spain through at the expense of England.

On 29 April 1954, Zarra received a tribute and benefit match in Madrid (played between two regional selections) in recognition of his long career in Spanish football.

This event was attended by several VIPs from the world of football, including Alfredo Di Stéfano, Antonio Puchades, Estanislao Basora, Piru Gaínza, Eduardo Manchon, and Cesar Lesmes.