In 878 Afonso III the Great occupied Coimbra and proceeded to repopulate the Mondego line; on December 2, 990 there was another onslaught of Arabs led by Almançor, who take the castle of Montemor-o-Velho, and its government is given to Froila Gonçalves, a descendant of the Portucalense count Gonçalo Moniz.

After new Arab attacks, Fernando Magno, in 1064, definitively conquers Coimbra and the Mondego line, giving its government to Count D. Sisnando Davides, a Mozarab native of a settlement located in the vicinity of Montemor-o-Velho called Tentúgal.

Already in the period of nationality, Sancho I left Montemor-o-Velho in testament to his daughter, Teresa, who, with her sister Sancha, gave charter to the village in May of 1212.

It grew, following a demographic boom throughout the Lower Mondego, with the consequent clearing of new land for cultivation and creation of new settlements, as can be seen in the toponymy which records numerous Casais and Póvoas.

It was in the castle citadel that king Afonso IV of Portugal and his advisors met on January 6, 1355, to decide the fate of Inês de Castro.

After the siege of Torres Novas, the Master of Avis, João, passed through Montemor-o-Velho, where he received the honors of the mayor and the people, and went on to Coimbra where he was acclaimed king by the Cortes.

On August 20, 1516, king Manuel I of Portugal gave a new charter (foral) to Montemor-o-Velho; this document is of particular importance for the analysis of the way of life of the people of Montemor at the beginning of the 16th century.

It is also at this time that there was an important change in the local economy due to the introduction of maize corn in the fields of the Mondego, brought from The Americas, which led to an era of prosperity that lasted until the seventeenth century.

In 1826 the municipality was made up of the parishes of Alfarelos, Brunhós, Carapinheira, Figueiró do Campo, Gatões, Gesteira, Granja do Ulmeiro, Liceia, Vila Nova da Barca, Alcáçova, S. Miguel, S. Salvador, S. Martinho and Madalena.

[6] Since 2014, the city hosts the Festival Forte that takes place inside Montemor-o-Velho Castle, in August, with the main focus on electronic music, visual and performing arts.