According to Mahābhārata (Mahabharata)and Manu, Amvaṣṭha (Amvashtha) is the hybrid child born of the womb of a Vaiśya(vaisya) woman, and sired by a Brāhmaṇa (Brahmana) man –
brāhmaṇād vaiśyakanyāyāmvaṣṭho nāma jāyate.

They are quite ancient as a jāti (jati), since they are mentioned in Aitareya Brāhmaṇa (Aitareya Brahmana). Maharṣi (Maharshi; Great Sage)Parvata and Devarṣi (Devarshi; the Divine Sage) Nārada (Narada)completed the Aindra Mahābhiṣeka (Mahabhisheka)of one Amvaṣṭha king.
aindreṇa mahābhiṣekena parvata-nāradau āmvāṣṭhyam abhiṣiṣicatuḥ.

Ptolemy has possibly referred to these people, by the term ‘Ambastai’. As it is found from the Greek sources, especially as Arian, during the invasion of Alexander, placed the ‘Abstanois’ on the bank of the river Asiknī(Aceesines), it appears that these people dwelt in the Chenub-Asiknī region of northern Punjab.
So, the Amvaṣṭha(s) lived in the lower portion of Mālava (Malava) and at the upper portion of the conjunction of Sindhu and Chenub. It appears that the modern district of Sikarpur was the habitation of the Amvaṣṭha people.
In Mahābhārata, the Amvaṣṭha(s) have been mentioned along with Śivi (Sivi), Kṣudraka (Kshudraka), Mālava and other north-western communities like Kaukura, Tārkṣya (Tarkshya), Pahlava, Mālava and so on.

Nakula, the fourth Pāṇḍva defeated the Amvaṣṭa(s) before the Rājasūya yajña. In the concluding ceremony of the yajña, they brought gifts for Yudhiṣṭhira.

In several incidents of Mahābhārata, the Amvaṣṭha(s) have been mentioned. For instance, one Amvaṣṭha king always stayed by the side of Bhīṣma. When, on the tenth day, Bhīṣma was alone in the battlefield, even then this Amvaṣṭha king did not leave his side. We have also found the presence of Amvaṣṭha warriors within the army of Bhīṣma.

During the generalship of Bhīṣma, one Amvaṣṭha king was defeated by Arjuna. During the generalship of Droṇa, one Amvaṣṭha king was seen at the backside of his Garuḍavyuha. On the fourteenth day of the war, one Amvaṣtha warrior got killed, another one was slain by Bhīma.

As found in several Purāṇa(s), the Amvaṣṭha(s) were associated with a king called Suvrata, in the genealogical line of Śivi, belonging to the lineage of Anu, born of the womb of Śarmiṣṭhā (Sarmistha), and sired by Yayāti (Yayati).

According to Bhāgavatapurāṇa , this Amvaṣṭha Suvrata sought to marry Lakṣmaṇā (Lakshmana), daughter of Vṛhatsena (Vrihatsena), but he could not fulfil the condition of the Svayamvara, that is, penetrating the eye of the fish. Later, Lakṣmaṇā was married to Kṛṣṇa.

In his Aṣṭādhyāī sūtra (Ashtadhyayi sutra) [8.3.97], Pāṇiṇi (Panini) has mentioned the Amvaṣṭha(s), and as the commentator Patañjali has commented [4.1.171], it appears that they followed the monarchic order. In Vārhaspatya Arthaśāstra (Varhaspatya Arthasastra), the Amvaṣṭha(s) have been mentioned along with Sindhu. From all these proofs, it is assumed that the Amvaṣṭha(s) were some warrior tribe of north-western India. However, after the fall of the Magadha empire, foreign invasion became rapid and rampant in north-western India, they got dispersed over different parts of India and resorted to such vocations as agriculture, medical profession, tannery and washing. We have also seen one Amvaṣṭha as the driver of an elephant. Before slaying Kaṃsa (Kamsa), when Krṣṇa came to Mathurā (Mathura), accepting the invitation for wrestling, Kaṃsa sent a crazy elephant called Kuvalayāpīḍa (Kuvalayapida). The man who looked after this elephant , was an Amvaṣṭha. He also got killed by Kṛṣṇa.

The Amvaṣṭha(s) were the predecessors of the Kāyastha(s) (Kayastha) in Vihar and Bengal, in modern times. In some Buddhist text, one Amvaṣṭha was referred to as a Brāhmaṇa, but they were mostly known as Kṣatriya (Kshatriya) — and it was proved by their relation to the Ānava Kṣatriya(s), Uśīnara(Usinara) and Śivi. In Mahābhārata , too the Amvaṣṭha(s) have been defined as Kṣatriya(s)
Śrutayu, a king of the Amvaṣṭha community, has been referred to as one warrior among the Kṣatriya(s)–
śrutāyurapi cāmvaṣṭhaḥ kṣatriyāṇāṃ dhurandharaḥ
[See Śrutāyu]