Prior to the imperial period Janus figures guarded property lines among neighbouring tribes, each of which had separate ianus. Originally one face was bearded while the other was not (probably a symbol of the sun and the moon). Later both faces were bearded. In his right hand he holds a key. The double-faced, bearded head appears on many Roman coins, and around the 2nd century BC in the time of Hadrian, Janus was depicted with four faces.

All doors and gates were sacred to Janus. Janus presided over the past, present, and future, and over war and peace. Janus represents the transition between primitive life and civilization, between the countryside and the city and the growing-up of young people.

The god of beginnings and all activities related to beginnings was very important to the Romans, because if undertakings had unsuccessful beginnings, they would lead to failure. Janus was worshipped at the beginning of the harvest time, planting, marriage, birth, and other types of beginnings, especially the beginnings of important events in a person's life. Janus was named first in prayers to the gods and received the first part of every sacrifice, he was the first god invoked, taking precedence before Jupiter. His name is invoked when sowing grain, as sowing represented a beginning. His blessing was requested at the beginning of every day, month, and year. As the most ancient of kings, Janus is supposed to have given the exiled Kronos a warm welcome in Italy, and to have offered Kronos a share of the royal duties.

One myth states that Janus came from Thessaly and that he was welcomed by Camese in Latium, where they shared a kingdom. They married and had several children, among which the river god Tiberinus (after whom the river Tiber is named). When his wife died, Janus became the sole ruler of Latium. He sheltered Saturn when he was fleeing from Jupiter. Janus, as the first king of Latium, brought the people a time of peace and welfare; the Golden Age. He built a citadel for himself on the Janiculum in Rome. His worship existed as a cult prior to the foundation of Rome and was introduced there by Romulus. Janus introduced the Romans to law, coinage and the cultivation of the fields. After his death he was made a deity and became the protector of Rome. His image is the most popular one used on coins. The highest of Rome’s seven hills, the Janiculum was named for him. Janus was a passive god, however in one legend, when Romulus and his associates stole the Sabine Virgins, the Sabines attacked the city and captured the Capitol of Rome. The daughter of one of the guards on the Capitolian Hill betrayed her fellow countrymen and guided the enemy into the city. They attempted to climb the Janiculum hill but they were kept from entering the Forum by fountains of boiling water that gushed from the temple's statue of Janus, the would-be attackers fled. Ever since, the gates of his temple were kept open in times of war so the god would be ready to intervene when necessary. In times of peace the gates were closed.

The sacred temple of Janus at the Forum in Rome served as an armoury and had two doors, one facing east and the rising sun, the other the setting sun. Inside the statue of Janus has one face looking out each door. When war was declared, the portals to the sanctuary of Janus on the Forum were opened. The portals ( a double barbican gate) were closed on the declaration of peace. During Rome's first 700 year history, the doors were closed only four times before the Christian era. Opening the doors may be symbolic of the way drawbridges were opened in time of war to protect the early city of Rome. His most famous sanctuary was a portal on the Forum Romanum through which the Roman legionnaires went to war. He also had a temple on the Forum Olitorium, and in the first century another temple was built on the Forum of Nerva. This one had four portals, called Janus Quadrifons. When Rome became a republic, only one of the royal functions survived, namely that of rex sacrorum or rex sacrificulus. His priests regularly sacrificed to him.

The month of January (the eleventh Roman month) was named after him. When the calendar was expanded the modern calendar’s new first month was named in his honour. Medieval writers accepted Janus as the New Year god. The beginning of the day, of the month and of the year was sacred to Janus. The festival, Agonia, was celebrated in his honour on January 9 and December 11.

In his capacity as porter he is depicted holding a staff in his right hand and a key or keys in his left. In later times he is depicted as bearded and unbearded and in place of the staff and keys the fingers of his right show the number 300 and his left show 65, the remaining days of the year.

Janus is referred in the 1995 movie adaptation of Judge Dredd. Janus is the name of the project where the DNA of all elder judges have been compounded to yield perfect Judges. However the result yielded is the perfect judge Dredd, and his DNA twin, the perfect criminal, Rico.

In the James Bond film GoldenEye, the renegade 00-agent Alec Trevelyan secretly formed the "Janus Syndicate" with himself ("Janus") as its head. This alludes to his betrayal of MI6, as Bond remarks "so you became Janus, the two-faced Roman god come to life."

In the film Syriana, the CEO of Connex who attempts to convince the Iranian princes into granting oil contracts is named Leland Janus, suggesting his two-faced nature.

In the Buffy the Vampire Slayer episode Halloween, Janus was the god Ethan Rayne called on to transform everyone into whatever they were dressed as.

In the video game Vampire: The Masquerade - Bloodlines, the Malkavian character addresses both "twin" sisters as "daughters of Janus". It is later proven that the sisters are in reality the same person, but with two personalities.

The Chrono Trigger character Magus's original name as a child was Janus.

Janus appears as a boss in the game Shadow Hearts: Covenant.