In 1909, in the chapel of the House of Armies, he married Raquel Sánchez-Benavides La Rosa, with whom he had seven children, including General Ernesto Montagne Sánchez.
In 1918, he was promoted to the rank of lieutenant colonel and was appointed head of a section in the General Staff of the Army; but shortly afterwards he was transferred to the Military School to take charge of his subdirectorate.
When Leguía was overthrown, he did not recognize the cabinet formed in Lima headed by General Ponce, on August 24, 1930, and rather supported the putsch led in Arequipa by Lieutenant Colonel Luis Miguel Sánchez Cerro.
Cerro immediately moved to Lima on the 27th of August and organized a new Government Junta, in which Ernesto Montagne was appointed as Minister of Foreign Relations.
When the conflict with Colombia broke out as a result of the occupation of Leticia by a group of Peruvians who were discontent with the Solomon-Lozano Treaty, Montagne was transferred to Iquitos as chief of staff of the Army foe Operations of the North-East in 1933.
Initially, his cabinet was predominantly filled with Army officers, Later, from October 1937, important civilian figures joined him, such as Dr. Carlos Concha (Foreign Relations), Benjamín Roca (Finance and Commerce) and Rafael Escardó (Public Health, Works and Social Welfare).
Once again elected senator from Loreto in 1945, he supported President José Luis Bustamante y Rivero and, together with lawmakers of the National Democratic Front, tried to contain and mitigate the opposition, APRA.
The Bustamante government was overthrown after the coup d'état of General Manuel A. Odría on October 27, 1948, who formed a Military Junta and called for presidential elections in 1950.
Odría then implemented his famous "descent to the plain": with barely a month to go before the elections, he left power to General Zenón Noriega (June 1950).
The government blamed the National Democratic League for the revolt and Montagne was arrested under various pretexts, although he was released after two weeks.