One of the oldest Etruscan cities, the original settlement was in the area known as La Civita, on the site of prehistorical settlements (Villanovian Age, 9th Century B.C.), which grew in size to rival that of Athens. It reached its peak in the 8th Century B.C., when it ruled over the nearby Monti della Tolfa (with their mines), before these passed to the nearby Etruscan city of Caere.
Thanks to its strategic position (near the sea and dominating the Marta valley) Tarquinia ruled both sea and river traffic through its port, Gravisca, growing rich on agriculture and trade, favoured also by close links with Greece and the Middle East.

As the city grew in population, so did its need for cemeteries. The first tomb in the Ara della Regina dates back to the 6th Century B.C., followed by those in the main necropolis of Monterozzi.
Such was the power of Tarquinia at this time, that two of the original seven Kings of Rome came from here (Tarquinius Priscus and Tarquinius Superbus). The fortunes of the city then began to fall, together with that of Greece. In 308 B.C. the city was finally defeated by Rome, seen as the start of a new period of prosperity. However, by 205 B.C., when Scipio asked for a contribution towards his campaigns in Africa against Hannibal, all Tarquinia could offer were a few sails.
Tarquinia became a Roman municipium in 90 B.C. During the Roman Empire, Tarquinia was basically a military post, controlling a section of the Via Aurelia (from Rome to France). In the 4th Century A.D. it became a bishopric, but the many Barbarian invasions, malaria and the fall of the Roman Empire led to a huge decline in the local population.

In the 6th Century A.D., for reasons still unknown, the population moved to the hill known as Corneto, where the town of Tarquinia is today. The first documents proving this are dated 649, 743 and 861. The city walls were built in the Mediaeval Era, as trade improved and thus the fortunes of the population. In the second half of the 11th Century the town belonged to the Countess Matilde di Canossa. The town grew rich on agriculture and sea trade, to the point that it even became an ally of Pisa and had profitable dealings with Venice, genoa and Ragusa. The towers of Tarquinia date back to this, the city-state period: originally 38, some 18 still rise above the skyline today.
Tarquinia was besieged by Frederick II of Svevia in 1245 and again by the Church in 1283. The inhabitants then rebelled against the governors Matteo Vitelleschi and Cardinal Albornoz, survived a seige by the Bretons in 1393. The population of Corneto gradually rose to 35,000 over the ensuing years and the town numbered more than 50 churches and 7 hospitals.

One member of a local noble family, Cardinal Giovanni Vitelleschi, became Commander of the Papal Army and fought the invaders along the coast. In his honour, the Roman Senate in 1418 granted Roman citizenship to the population of Tarquinia. The city walls were strengthened by the cardinal and the Palazzo Vitelleschi was built, now home of the National Etruscan Museum of Tarquinia.

The plague hit Tarquinia in 1478: to repopulate the area families from Albania and Lombardy were brought in! Then the plague of Rome in 1492 forced Pope Alexander VI to take refuge in Tarquinia.
The city became part of the Estate of San Pietro in Tuscia in the 16th Century (i.e. it was annexed by the Church). Occupied by the French in the late 1700s and even by the British, it returned to the Church after the Congress of Vienna.
Tarquinia was annexed to the Kingdom of Italy in 1870 with the name of Corneto Tarquinia (changed to plain Tarquinia in 1922) and hosted Garibaldi for three days in 1875.

Only in relatively recent times has the city rediscovered its Etruscan past.