In August and September of 1937 he carried out the first military officer's course at the Machine Gun Barracks (Quartel de Metralhadoras I) in Lisbon, which extended to a second course, from August to October 1938. During that academic year, he taught at the Escola Industrial de Marquês de Pombal in Lisbon, and in late 1938 he returned for the second year of teaching practice at Liceu Pedro Nunes, where he later passed the state examination for teachers, with the defence of a research paper on Sá de Miranda, obtaining a final score of 16 points.

His teaching career led him to many parts of the country, including: Santarém, where he taught at Liceu Nacional; Braga, at the Liceu Nacional Sa de Miranda; Lisbon again, at the Liceu Nacional Passos Manuel; and Lamego, at the Liceu Latino Coelho, where he became a permanent teacher in February 1940. That same year his teaching activity was suspended once again due to mandatory military service, first in Mafra and then in Lagos.

In 1941, according to the Portuguese Government Gazette of 6 July, he received a grant to 'assist Dr. Leite de Vasconcelos' in his literary work, but did not end up using it, continuing nevertheless to work actively with his professor. That same year, he accompanied his sister in her move to Lisbon, returning to the city and helping her finance her studies. In March he took over the professorship at the Military College, where he worked until September 1944, teaching simultaneously at the Colégio Infante de Sagres school. That same year he moved back to the Algarve, establishing himself as a professor in the Liceu in Faro, where he remained until 1948, the year he became a teacher at the Liceu de Diogo Cão in Sá da Bandeira, Angola.

During the 1940s, influenced by his colleague and friend Agostinho da Silva (who at that time was in exile in Uruguay) he considered immigrating to South America, however his destination ended up being the African continent. On that first trip to Angola he became acquainted with Catholic missionary Carlos Estermann (1896-1976), a leading researcher on African ethnic cultures, in particular Angolan culture, and whose work formed a fundamental influence on the research of Viegas Guerreiro.

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