In the year 398 BC, during the siege of the city of Veii, the Romans dug a tunnel out of bare rock for one and a half kilometres in order to control the water level of Lake Albano: the Emissary is considered a great hydraulic engineering construction which is still visible today.

Already, in the Republican period, many elegant villas were built around the lake. The splendid Villa of Domitian built on the west side of the lake dates from the Imperial period. Today, the ruins are under the rule of the Vatican State.

As reported on the Papal bull in May 1037, Castel Gandolfo was identified as an independent agricultural community and remained so until the year 1000 by paying a rent to the Monastery. It is not known exactly when and how the noble family Gandolfi, after which the town is named, took possession of the area. However, it is known that at the beginning of 1200 a castle was built by the Gandolfi in the ancient village of Cuccuruttus (name of the village in Medieval times). It later became property of the Capizucchi and then of the Savelli. It was a fief of the latter until 1596 when it was confiscated by the Apostolic Chamber, owing to the huge family debt. In 1604, Pope Clement VII included the castle in the inalienable properties of the church and began using it as a summer residence for the Papacy.

Under the supervision of the Apostolic Chamber many public works were carried out; Pope Urban VII (1623-1644) commissioned the roads known as Via Galleria di Sopra and Via Galleria di Sotto that connect Castel Gandolfo to Albano; Pope Alexander VII (1655-1667) took care of the construction of Via Alessandrina, the arrangement of the Pontifical Palace in 1660 and the building of the Collegiata di San Tommaso da Villanova (Collegiate Church of St. Thomas of Villanova), designed by architect Gian Lorenzo Bernini; in 1728 Pope Benedict XIII (1724-1730) gave the governance pro-tempore of Castel Gandolfo to the Quartermaster-General of the Sacred Palaces who did so until 1870; lastly we owe the bell on the façade of the Pontifical Palace, the enlargement of the motorway 216 Maremmana III to Pope Benedict XIV (1740-1758).

The capture of Rome, on 20th September 1870, brought an end to the Papal State and the residence of Castel Gandolfo was abandoned until 1929 when, after the Lateran Treaty, Benito Mussolini gave the Pontifical Palace and the adjoining villas to the new Vatican State. In 1933, the Astronomic Observatory was transferred to the Pontifical Palace (the so-called Specola Vaticana).