The Changanassery municipal area is located at the southern tip of the Kottayam district in Central Travancore, spreading across three villages. Bordered on three sides by wet paddy fields (punja paadam), Changanassery has the appearance and structure of an island. There are multiple opinions regarding how this ancient and historic land got the name 'Changanassery'.

The first opinion is that the name 'Changanassery' was established because Chenkannan, a ruler of the Sangam period, governed this region for some time. However, this view is not very accurate, as the place name Changanassery is not found in copper plates or stone inscriptions formed between the 9th and 12th centuries. Another opinion suggests that ‘Sankaranarchery’, meaning the habitat of the Shaishavas (or followers of Lord Sankara), later became Changanassery. Similarly, it is also said that ‘Sankhanadassery’ eventually evolved into Changanassery.

Among these opinions, the most notable one is that ‘Changazhinazhiyuri’ transformed into Changanassery. It is believed that the name 'Changazhi Nazhiyuri' was given to a plot of land with a sowing capacity of a changazhi and a nazhi (ancient units of measurement), which was donated by the ruler of Thekkumkoor for the construction of a temple. Through colloquial usage over time, it reportedly evolved into Changanari and then Changanassery. If this is true, the place name took shape around the 15th century AD, as the name Changanassery is not found in any records prior to that period.

Before that time, the general name of Changanassery was Nanthuzhainadu. Places like Perunna, Poovam, Vazhappally, and Thrikkodithanam fell within the boundaries of this land. Nanthuzhainadu was not an independent, sovereign state; it was merely a vassal kingdom under the Kulasekhara Emperor. The local ruler (Naduvaazhi) governed Nanthuzhainadu in consultation with the 'Nanthuzhainadu Munnuttavar' (The Three Hundred of Nanthuzhainadu). This 'Munnuttavar' served as the 'village council' of that time, and the ruler could do nothing without their consent.

Apart from the Nanthuzhainadu Munnuttavar, another village assembly called the 'Vazhappally Oorar' existed in Changanassery, as evidenced by the Vazhappally inscription of 830 AD. The 'Oorar' can be considered the executive committee of the village assembly. Additionally, the inscription also hints at a 'Parishe Sabha' (an ecclesiastical council). All of these point to the ruling class of the Changanassery region, which was part of Nanthuzhainadu. The ruler of Nanthuzhainadu governed until the 12th century AD under the Kulasekhara Emperor with the assistance of the Nanthuzhainadu Munnuttavar, the Vazhappally Oorar, the Parishe Sabha, and the local inhabitants.

Nanthuzhainadu ceased to exist due to Chola invasions. The history of Changanassery from the 12th century AD to the 15th century AD remains shrouded in darkness.

The new earthen pottery pieces discovered during excavation work on the western side of the Changanassery Convithpuram School and within the Changanassery Government Hospital compound serve as evidence pointing toward the ancient civilization of Changanassery. The known history of Changanassery begins with the emergence of Nanthuzhainadu. Nanthuzhainadu was one of the eighteen provinces (naadukal) that existed during the reign of the Kulasekhara Emperors, who ruled Kerala from the 9th century AD to the 12th century AD with Mahodayapuram as their capital. The capital of that province was Thrikkodithanam. Although there is no clear evidence regarding the exact extent of Nanthuzhainadu, Changanassery was entirely included in this province. Nanthuzhainadu, including Changanassery, was an excellent agricultural region. The literal meaning of Nanthuzhainadu is 'the land of good cultivation'.