Ambasthas as a tribe existed at least as early as the time of the Aitareya Brahmana, when they were probably settled in the Punjab; and they can be traced up to the present day in parts of Bengal and Bihar, from where they migrated in later times. In the Aitareya Brahmana King Ambasthya is mentioned as having been consecrated with the Aindra Mahabhiseka along with nine other kings. The Mahabharata mentions the Ambasthas along with the Sivis, Kusudrakas, Malavas and other north-western tribes. In the Bhisma and Drona Parva, the Ambasthas are referred to as having taken part in the Kurukshetra war, on the side of the Kurus; while in the Karna Parva , Srutayuh, the valiant Kshatriya, who was killed by Arjuna, is described as a king of the Ambasthas. The Ambasthas were also once defeated by Nakula, the fourth Pandava, along with the Sivis, Trigarttas and Malavas and Srutayuh was among those who did homage to Yudhisthir after the defeat.

In the Indian Puranas, the Ambasthas are represented as Anava Kshatriyas, and are said to have originated from Suvrata, son of Usinara; they were thus intimately related to the Yaudheyas and the Sivis, and were settled on the eastern border of the Punjab. The country is mentioned in the Barhaspatya Arthasashtra where it has been associated with Sindh.

The Puranas seem to represent the Ambasthas as Kshatriyas, descended as they were from Usinara; and, as we have seen, that Mahabharata refers to their King Srutayuh as `the best of Kshatriyas`. But the evidence of Smriti literature seems to point to their mixed origin.

According to the Gautama-Dharmasutra children born of wives of the next, second or third lower castes become Sabarnas, Ambasthas, Ugras, Nisadhas, Dausyantas or Parasaras. The Ambasthas would thus be descendants of Brahmanas by Kshatriya, Vaisya or Sudra wives.

In the Ambattha Sutta an Ambattha is called a Brahmana; but, according to the Jatakas, the Ambasthas were farmers, while Manu describes them as a people who practised the art of healing. Some historical reference says that Ambasthas were a tribe who in the beginning were a fighting race but in the later ages they took to different professions like being priests, farmers, writers and physicians.

History says that tribe seems to have migrated eastwards as well because even today a class of Kayasthas known as Ambastha Kayasthas can be traced in Bihar; while the Vaidyas of Bengal claim to be designated as Ambasthas.