When the Romans conquered the area, it was primarily occupied by Pannonians, Dacians, and Celts. Pannonians were probably Celticized, but not originally Celts. The Dacians are related to Thracians and are the ancestors to modern Romanians. In the fourth century CE, the Romans allowed "barbarian" tribes, including Goths, Alans, Huns, Marcomanni, and others, to settle in Pannonia (modern day Hungary). Many of them left this region for Western Europe, allowing the Huns to expand their empire. After the Hunnic Empire was defeated, more Germanic tribes (including the Gepidi and Ostrogoths) moved into Pannonia, which brings us to the end of the Roman Empire.

In the fifth century, the Ostrogoths left and Slavs began to move in. Lombards moved in as well, but were pushed out by the Avars. They formed their own empire, which lasted until the early ninth century. Parts of it were taken by the Bulgarians and, after a time, it became tributary to the Franks. Finally, the Magyars invaded, leading to their establishment in the Hungarian plain.

So, before the arrival of the Magyars, there was a number of peoples in the Hungarian plain. It was dominated by the Avars, Bulgarians, and Franks, but there were still groups of Germans and Slavs in the area as well.