Tonga is a Tumbuka offshoot Bantu language that emerged in 18th Century when the Nkhamanga Kingdom started to decline and was split. Before the arrival of missionaries in what is now known as Malawi, Tonga was the Tumbuka dialect. It was after the missionaries established their churches when they treated the two as separate languages.

Tonga is grouped in the Glottolog classification along with Tumbuka in a single group.

The Tonga language as a legacy offshoot, has been described as "similar" to Tumbuka, and Turner's dictionary (1952) lists only those words which differ from the Tumbuka.

Malawian Tonga is classified by Guthrie as being in Zone N15, whereas the Zambian Tonga is a different language classified as Zone M64. Therefore, the two languages are not the same but they only share a similar name.

Almost all verbs found in Tumbuka are all available in Tonga. However, there are few slight differences that from an outsider, it may be hard to notice. The two languages are mutually intelligible . Below is paragraph in Tonga and Tumbuka with English translation showing their slight differences:

Fumu ya boma la Nkhata-Bay yapempha mulungozgi wachalu chinu kuti wachitepu kanthu po mitengu ya vinthu iyu ilutiliya ku kwera.

Fumu ya boma la Nkhata-Bay yapempha mulongozgi wachalu chino kuti wachitepo kanthu pa mitengo ya vinthu iyo ikulutilila ku kwera.

The chief of Nkhata-Bay District has begged the president of this country to do something about the prices of things that keep on rising.