The first reference to the idea of afforesting the Serra de Monsanto appeared in 1868, in a report on the general afforestation of the country by Carlos Ribeiro and Nery Delgado:
“...we can remind you of the outskirts of Lisbon, the entire Serra do Monsanto, which would be very useful for afforestation, so as to later supply the Capital with wood and timber, while at the same time alleviating the aridity that travellers notice when they enter the Tagus, seeing on either side steep mountains (...) the green masses of trees that would give the City a brighter appearance and favourably modify the climate, effectively contributing to public health.”.

Later, in 1926 and 1927, the planning proposals presented by Forestier and Mac-Bride respectively considered afforestation and the installation of enclosures for games and sports, as well as leisure facilities with lakes, viewpoints and seating areas, in the north-west sector of the Serra de Monsanto.

However, it would take the strong political will of the Estado Novo, well defined by Mr Duarte Pacheco (Minister of Public Works in 1932 and Mayor of Lisbon in 1938), for the Forest Park to become a reality.

The park, then known as Parque Florestal (Ocidental) da Cidade, was involved in the final stretch of the Estádio Nacional - Lisboa motorway. Destined to be the city's “green lung” and a leisure area for the population, it contrasted with the Jamor Valley (forested during the same period), but where there was a lot of sports equipment.

Once Lisbon's city planning had been defined through the GROER plan, a project for the city's Forest Park was carried out by architect Keil do Amaral, a project that began in the 1940s but was never fully implemented.

The spaces left free for the planned equipment and infrastructures ended up being occupied by facilities of the Portuguese Air Force and other institutions, definitively rendering the park's initial project unfeasible.

Legally created by a Decree-Law in 1936 (1), the Park was subject to the Total Forestry Regime in 1938 (2), the date from which an accelerated regime of expropriations (and appropriations) of the land took place, along with the afforestation and installation of the planned equipment.