[1] He still is the most successful president in the history of the club; under the guidance of Fred Pentland, Athletic won two La Liga titles and four Copa del Reys in his four years in office.
[4] He had an older brother Carlos (1881–1903), who became the first president of Bilbao FC, while one of his cousins, María Mestayer Jacquet (1877–1949), known as "Maritxu", became the first gastronomic journalist in the Basque Country and probably in Spain.
[5][3] During his brother's comings and goings to the United Kingdom, he had the opportunity to learn first-hand about the successful growth of professional football, and so, through the correspondence he maintained with Manuel, he recommended him the convenience of becoming a club,[5] and thus, on 30 November 1900, Bilbao Football Club was officially established in an informal meeting held in the house of industrialist José Luis de Villabaso,[3] who knew his father Manuel Castellanos since at least 1891 when they were members in the first board of directors of Banco del Comercio [es].
[1][10] Thanks in part to the great management of Fred Pentland, Castellanos became the most successful president in the history of the club, with two La Liga titles and four Copa del Reys in his four years in office.
[6] Another son, Manuel, was less fortunate; enlisted in the Spanish Civil War on the day nationalist troops entered Getxo, he perished on the front just 24 hours later, so his father named his next child after him.