Monjardino attained his Licenciatura in Law, as well as the Curso Complementar de Ciências Jurídicas (Complimentary Course in Judicial Sciences) before beginning his career in politics.
Monjardino was elected as an independent deputy for the National Assembly in the Estado Novo regime, under the banner of the ANP, for the district of Angra do Heroísmo, between 1973 and 1974.
Monjardino was described as a historian and studious investigator in judicial matters, publishing various documents on the subject.
At one time he was president of the council of the Instituto Histórico da Ilha Terceira (1984–1999), as well as a correspondence partner of the Portuguese Academy of History.
He was one of the principals involved in the classification of the historical centre of Angra do Heroísmo as a World Heritage Site by UNESCO.