Mills was one of the most important footballers in the amateur beginnings of Athletic, co-founding the club and then serving as a board member and a captain, winning the 1904 Copa del Rey.
[3] At some point in the early 1890s, Mills arrived in Bilbao with his father to work for the cable company that connected Spain and Great Britain by telephone.
[3][6][7] Mills was the only foreign-born person among the 33 who signed the documents and the club's only foreigner in its ranks,[6][7] and thus he became known in Bilbao simply as "El Inglés".
[3][8] In addition to being a founding member and a club captain, Mills was a regular starter on the pitch too, playing as a defender who could also operate in midfield.
[3][11] In the 1904 Copa del Rey Final, Athletic was declared winners again without playing a single match since their opponents failed to turn up.
Although he lived in Logroño, an attack of uremia made him move to Algorta, where he continued to work for the telegraph company until he passed away on 15 October 1929, at the age of 54.