He led a campaign to conquer southern Gaul against the Allobroges together with his successor Quintus Fabius Maximus Allobrogicus.

Domitius was active in the early development of southern Roman Gaul, establishing the first Roman colony at Colonia Narbon Martius, and sponsored projects such as the Via Domitia connecting Italy to Spain through southern Gaul.

[2] In the 120s BC Rome had become increasingly active in Mediterranean Gaul, launching three separate campaigns into the region in the years immediately before Ahenobarbus' consulship: in 125 to protect Rome's ally Massalia from the Salluvii, in 124 BC Rome seized the capital of the Salluvii (the city of the Gauls near the oppidum of Entremont, near modern Aix-en-Provence), and in 123 BC again defeating the Salluvii and establishing a permanent garrison for Roman troops at a nearby place hence called Aquae Sextiae (modern Aix-en-Provence).

The Allobroges did harm to the pro-Roman Aedui tribe and harbored a leader of the Salluvii, King Teutomalius, who escaped the destruction of his city and enslavement by the Romans from the previous year.

At the same time, consul Quintus Fabius Maximus Allobrogicus led another force to the north near modern Valence at the convergence of the Rhône and Isère rivers where they achieved a resounding victory over a significantly larger force of Celts, suffering minimal casualties.

[4] Having completed the conquests of what is now in western Languedoc, Domitius established the first Roman colony in Gaul called Colonia Narbo Martius (Narbonne) in 118 BC.

It was built along an ancient trading road, crossing the Alps by one of the easiest passages, the Col de Montgenèvre.