In the legend referred to by Livy the Campus Martius was originally a wheat field owned by Tarquinius Superbus. Burned during the expulsion of the Tarquins, it then became property held in common by the Roman People. The Campus itself was a low-lying plain enclosed on the west by a bend of the Tiber River, on the east by the Quirinal Hill, and on the southeast by the Capitoline Hill. Although close to the city and the Forum Romanum, the field lay outside the Servian Wall and the sacred pomerium. It was, therefore, an ideal location for pasturing sheep and horses and for military training activities.

The Campus' use for the purpose of military training led to it being dedicated to the god Mars, protector of the Roman Soldier. The Campus Martius also contained the Temple to Bellona, Roman Goddess of War.

Because it was outside the city walls, it was also an ideal place to meet and house foreign delegations, to locate foreign cults, and for Roman commanders to await Senatorial approval of their triumphs.

Over the centuries Roman statesmen added to the buildings on the Campus. In 221 BC Gaius Flaminius Nepos added the Circus Flaminius to the southern side, next to the Tiber River. It was during the Dictatorship of Sulla that the first building lots along the Campus Martius were sold or donated to influential Romans, and private homes began to encroach on the public land.

Gnaeus Pompeius constructed the first stone theater in Rome along the Campus Martius in 55 BC. The same building contained a Senate meeting house as an annex, and it was in that building that Julius Caesar was assassinated.

Caesar had planned to move the Saepta, the enclosures for electorial voting, to the Campus; by the time they were completed by Augustus Rome's elections were effectively moot.

Other buildings later constructed on the Campus Martius include:

*The Porticus Octavia, constructed from the spoils of the Dalmatian War.
*The Ara Pacis (Altar of Peace)
* The Porticus Argonautarum, the Laconicum Sudatorium, the Thermae Agrippae, the Pantheon and the Aqua Virgo, all initially donated by Marcus Agrippa. The Pantheon was later reconstructed by the Emperor Hadrian.
*The Mausoleum of Augustus
*The Theater of Marcellus
*The Temple of Isis

The Campus Martius was enclosed within the Aurelian Walls c. AD 270 and, due to its location next to the Tiber River, remained a highly populated area even after the collapse of the Roman aqueduct system.

City Builders: Heraklia Aelius Senex Caecilius Notable Residents: Fabricius Flavius M. Fabius Furius QuintusCinna Cocceius