In 1917, soon after graduating, Muguruza started to work as lecturer at the School of Architecture thanks to a proposal by Ricardo Velázquez Bosco, and, in March 1920, he finally obtained a Chair of "Projects of Architectural and Ornamental Details".[6]
He married Mercedes Peironcely y Puig de la Bellacasa in 1921. They had no issue.[7]
During the Second Republic he authored some markets, such as Santa María de la Cabezas's (1933) or Maravillas (1935).[8]
After the outbreak of the Civil War in 1936, Muguruza fled from the Republican area and joined the Francoist side.[12] Franco entrusted him the task or reorganizing the architecture in the territory controlled by the rebels.[12] Muguruza assumed as member of the Real Academia de Bellas Artes de San Fernando in 1938.[13] In June 1939, only 3 months after the Francoist victory in the war, he presided over the Assembly of architects in Madrid, setting the ideological foundations behind the architecture of the new regime.[14] Already Chief of the Services of Architecture of FET y de las JONS,[14] he was appointed to the leadership of the Directorate General for Architecture, structured along totalitarian lines.[15] Muguruza served in the post from 30 September 1939 to 8 March 1946.[16]
Muguruza and his disciple Diego Méndez were the architects who designed the Valle de los Caídos;[17] they aimed to make the site an eternal metaphor of the regime's ideology.[17] He directed the building works until leaving in 1949, reportedly because of a degenerative paralysis; he was replaced by Méndez.[8]
Delso, Rodrigo; Amann, Atxu; Soriano, Federico (2019). "Time, Architecture and Domination: The Valley of the Fallen". Heritage & Society. 11 (2): 1–25. doi:10.1080/2159032x.2019.1670534. S2CID211657577.