On 5 September 1901, Amado Arana was one of the 33 members who signed the documents that officially established the Athletic Club at the historic meeting in Café García (Mario did not do so as he was only 17 at the time).
[3] He was then one of the first players of the newly created team, with whom he played several friendly matches against city rivals Bilbao Football Club at the Hippodrome of Lamiako.
[5] Three weeks later, on 31 March 1902, he was again in Bizcaya's starting XI for the return fixture at Lamiako, the first visit by a foreign team to Bilbao, helping his side to a 7–0 win.
[7][8] He was also part of the Athletic team that won the 1903 Copa del Rey, featuring in the final alongside the likes of Alejandro de la Sota, and club founders Juan Astorquia and Eduardo Montejo;[9][10] Arana was at the heart of a 3–2 comeback win over Madrid FC (now known as Real Madrid) in the final.
[9][10] He was also part of the team for the 1904 Copa del Rey, which Athletic won without playing a single match since their opponents failed to turn up.