Cossus was a member of the patrician gens Cornelia. The gens Cornelia was one of the most important families of the Roman Republican period having first attained the consulate in 485 BC and remaining prominent throughout the next four hundred years, producing figures such as Scipio Africanus and Lucius Cornelius Sulla. Aulus Cornelius Cossus Arvina belonged to the Cossi branch of the Cornelia gens, which was descended from Aulus Cornelius Cossus, a warrior of the 5th century BC who had personally slain Lars Tolumnius the King of Veii, becoming one of only three Romans in recorded history to acquire the honor of the spolia opima. Judging by his filiation it seems likely that Publius Cornelius Arvina, who was consul in 306 and 288 BC, was his son.^[1]

Cossus first appears in history in 353 BC as the Magister Equitem serving under the dictator Titus Manlius Torquatus. The purpose of Manlius' appointment as dictator was to combat the city of Caere, an Etruscan city whom had recently allied with the city of Tarquinia against Rome. Upon hearing of Rome's appointment of a dictator, the Caerites decided to surrender and a one hundred year long truce was negotiated between the two cities. The campaign was then redirected against the Falisci who were also accused of conspiring against Rome, however the Romans were unable to locate the Faliscan army and instead decided to ravage their territory.^[2] The role that Cossus played in these events is unrecorded.

In 349 BC, Torquatus was appointed dictator for a second time, and once again Cossus was appointed as magister equitum. The purpose of the appointment of Torquatus as dictator was to hold the consular elections for the following year, with one of the consuls elected being the young Marcus Valerius Corvus, consul for the first time.

In 343 BC, Cossus was elected consul for the first time, serving alongside Marcus Valerius Corvus, now in his third consulship. It was in this year that the First Samnite War broke out over a dispute between Rome and the Samnites over the city of Capua. In reaction to this, the senate assigned his colleague Valerius with the responsibility of defending Capua from Samnite attacks, while Cossus was ordered to march into Samnium itself.^[3] The campaign started badly, as Cossus unwisely marched his army into a ravine which was beset on both sides by hostile Samnites, only realizing his mistake after it was too late to retreat. Publius Decius Mus, a military tribune serving under Cossus, contrived an escape plan and received the commander's permission to execute it. Decius took a small detachment of men and seized the summit of an unoccupied hill, starting a fight which distracted the Samnites and allowed Cossus and the rest of the army to escape.^[4] Through their own valor and the confusion and exhaustion of the enemy, Decius and his men escaped from the hill and put the Samnites to flight, despite being very outnumbered. Cossus and Decius then rejoined their forces and managed to run down the fleeing Samnite army, scoring a decisive victory.^[5] To thank Decius for his service, Cossus presented to him many valuable gifts, while the army awarded Decius the Grass Crown, an extremely prestigious and rare Roman military honor granted to a soldier who had saved an entire army.^[6] As a reward for his defeat of the Samnites, Cossus was granted a triumph by the senate, one of two triumphs celebrated that year, as his colleague Marcus Valerius also celebrated a triumph for his own victories against the Samnites.

In 332 BC, Cossus was elected consul for a second time, with Gnaeus Domitius Calvinus as his colleague. His election was secured under the auspices of Marcus Valerius Corvus, his former consular partner, serving as interrex. In this year Marcus Papirius Crassus was appointed dictator as a reaction to rumors of a gallic invasion. However, these rumors turned out to be unfounded and the dictator resigned. The rest of the year was quiet, the most significant event being a treaty negotiated between Rome and Alexander of Epirus on the basis of their common hostility to the Samnites.

In 322 BC, in the midst of the Second Samnite War Cossus was appointed as dictator in order to fight off the Samnites, who were rumored to have strengthened their armies with a large number of mercenaries. As dictator Cossus appointed Marcus Fabius Ambustus as his Magister Equitem, and went to war. Cossus met the Samnite army soon after advancing on enemy territory. Since it was too close to sunset to commence battle, both sides set camp for the night. Cossus decided to retreat rather than confront the Samnites, but in the morning the enemy cavalry realized his intentions and moved to stop him. Soon the Samnite infantry caught up with the cavalry and set battle lines. Cossus' attempts to build a defensive camp were thwarted by the enemy, forcing him to commit to a battle. The Romans were determined and the Samnites were overconfident, and fighting continued for five hours without either side gaining an advantage. This stalemate broke when some of the Samnites left the field to loot the undefended Roman baggage train. On Cossus' orders, Marcus Fabius successfully drove off the looters, boosting Roman morale and depleting the morale of the Samnites. Reinvigorated, the Roman infantry pressed their attack and were joined by the cavalry, soon routing the Samnites. For this victory the Senate awarded Cossus a second triumph.^[7]

However, there is a second tradition which is imparted by Livy which maintains that this battle was in fact conducted by the consuls of the year, and the purpose of Cossus' appointment as dictator was to oversee the Ludi Romani in place of the praetor who would usually oversee the games, as a result of the given praetor of the year being very ill at the time.^[8]

The next year the two consuls Spurius Postumius Albinus Caudinus and Titus Veturius Calvinus were caught in an ambush by the Samnites and were forced by them to pass under the yoke as well as to agree to a humiliating peace treaty. The following year, 320 BC, after the two consuls returned to Rome, the former consul Postumius requested the senate to allow him and his colleague to submit themselves to the Samnites as a punishment for their defeat, escorted by a Fetial, a type of priest whom ritually presided over foreign treaties and affairs.^[9] The senate approved this action and appointed Cossus as the fetial to follow the two ex consuls to Samnium. Once arrived, Cossus offered the former consuls to Gaius Pontius, the Samnite commander who was responsible for the Roman defeat the previous year. Upon hearing what Cossus had to say, Pontius briskly refused his request, believing that this was an attempt to invalidate the treaty of the previous year.^[10] This incident is the final time that Cossus appears in our sources.

• Broughton, T. Robert S., The Magistrates of the Roman Republic, American Philological Association (1952)
• Livy (Titus Livius), Ab Urbe Condita Libri