After studying law, he passed the state exam to become a notary, and started supporting his mother and four siblings.
One of the drafters of the Constitutions of the Second Spanish Republic in 1931, and a member of the Radical Republican Party presided by Alejandro Lerroux, Hidalgo became Minister of War in 1934.
When serving as Minister, Hidalgo chose Francisco Franco (who was later going to be lead nationalist Spain during the civil war and be the caudillo for nearly 40 years) as one of his advisers.
During the Spanish Civil War (1936–1939), Diego Hidalgo had to flee from Spain, where he was threatened both by Nationalists and Republicans.
Notably, he saved 39 Republican political prisoners from being executed, proving their innocence.