As a young man he was a firm supporter of Basque nationalism, a contributor to the radical newspaper La Tradición Navarra and the editor of Euzkadi. In 1914, he staged his El jardín del mayorazgo theatre play, strongly anti-Spanish in content.
Aznar Zubigaray joined the Basque Nationalist Party in 1916, affiliating with its most radical wing. Since 1914, he had worked as a correspondent from World War I frontlines, which earned him the position of editor-in-chief at the central El Sol newspaper. In 1922, he and his family left for Cuba, where Manuel Aznar Zubigaray worked for several local newspapers, returning with the proclamation of the Second Spanish Republic (1931).
He moved towards Conservatism in the Republic years, and became a firm Franco supporter. In 1936, with the start of the Spanish Civil War, Aznar Zubigaray left for Burgos and offered his services to the Francoist army and the Falange. Throughout the war, he was an important author of propaganda and chronicler of military events. In 1940-1943, Aznar wrote his most important works, the Historia militar de la Guerra de España (1936-1939) ("Military History of the Spanish War") and Historia de la Cruzada ("History of the Crusade").
Between 1964 and 1967 he was Spain's ambassador to the United Nations. He also served this role in Morocco, Argentina and the Dominican Republic. He edited the La Vanguardia newspaper and helped start the EFE news agency. He is interviewed in the documentary film Franco, ese hombre, a biography of the Spanish dictator. He is the grandfather of José María Aznar.