He received his secondary education at the Colegio Nuestra Señora del Recuerdo. Martín-Artajo earned a law degree from the University of Madrid. He became a staff attorney of the Council of State in 1931. During the Republic, Martín-Artajo worked closely with Ángel Herrera Oria, the director of the Catholic newspaper El Debate and belonged to the lay "National Catholic Association of Propagators of the Faith" (propagandistas). With the start of the Spanish Civil War, Martín-Artajo went over to the insurgent Nationalists. He was a legal adviser to the Nationalist government's Junta Técnica del Estado (State Technical Council), Franco's pseudocabinet, and to the Nationalist government's Labor Ministry. In 1940, General Franco appointed him president of the mass movement, Catholic Action.
In 1945, Martín-Artajo participated in the drafting of the quasiconstitutional "Fuero of the Spanish People", a list of rights, freedoms, and responsibilities that was internationally regarded as a sham.
In July 1945, fresh after the defeat of fascist Germany, Franco wanted to present the Spanish government as "Catholic" rather than a profascist, in the face of ostracism from other Western countries. Franco wanted to appoint Martín-Artajo Minister of Foreign Affairs. After consulting with the primate of Spain, Cardinal Enrique Pla y Deniel, he accepted the portfolio of Foreign Affairs and resigned from his position at Catholic Action. His diplomatic efforts succeeded in breaking the dictatorship's isolation. He effectuated the signing of the Concordat with the Holy See in August 1953, the bilateral Pact of Madrid with the United States the following September, and Spain's entry into the United Nations in 1955.
After retiring from the Foreign Ministry, he worked on the Council of State and at the publisher, Editorial Católica.