Idukki District was formed on January 26, 1972, by carving out Devikulam, Udumbanchola, and Peerumade Taluks from the erstwhile Kottayam District, and Thodupuzha Taluk (except for Kalloorkad Village and a portion of Manjalloor Village) from the former Ernakulam District. The name "Idukki" is derived from the word "Idukku," which means "gorge."

On October 29, 1982, the North Pamba Valley area and the region around the Sabarimala Sannidhanam in Mlappara Village of Peerumade Taluk were transferred to Pathanamthitta District. Currently, Idukki District comprises five Taluks: Devikulam, Udumbanchola, Thodupuzha, Peerumade, and Idukki.

The pre-history of the District is unclear, but archaeological evidence points to the presence of Megalithic-era structures such as uru buriah, pandukuzhies, menhirs, dolmens, and graves found in the high ranges and hills. Excavations by the State Archaeology Department in Marayoor (Anchunad Valley), Thengakkal (near Vandiperiyar), Bison Valley, and Thondermalai reveal distinct characteristics of pre-historic civilization. The discovery of dolmens and menhirs in these areas suggests a well-developed culture during the Megalithic period. Additionally, the District played a significant role in the ancient spice trade, as it was known for abundant spices like pepper and cardamom.

Some historians believe that Kuzhumur, the capital of the early Chera Empire, is present-day Kumily in the Peerumade Taluk of the District. During the reign of the Kulasekharas, areas such as Nanthuzainad (including Devikulam, Udumbanchola, and Peerumade Taluks), Vempolinad, Manjunad (of Kottayam District), and Kuzhumelainad (including Thodupuzha Taluk) were part of the Kulasekhara Empire. Around 1100 AD, Vempolinad split into Vadakkumkur and Thekkumkur, with Karikkode in Thodupuzha Taluk becoming the capital of Vadakkumkur. For a long period, Vadakkumkur was subordinate to the Perumpadappu Swaroopam. The Thekkumkur Kingdom was powerful at the time, often engaging in conflicts with the Cochin and Vadakkumkur Kingdoms.

The Dutch East India Company was drawn to the high ranges of the District due to the abundant growth of pepper. On June 16, 1664, the company signed a trade agreement with the Thekkumkur Rajas covering spices, cinnamon, opium, and more. Meanwhile, Manavikramakulasekhara Perumal, the Pandya King, and his family fled to Kerala as refugees and settled in Vadakkumkur. With the help of the Vadakkumkur Rajas, the Pandya King acquired the land west of the Western Ghats, known as Poonjar, from the Thekkumkur Rajas. The Poonjar territory was transferred to the Pandya King, who was crowned there in the presence of the Thekkumkur Rajas. In the 15th century, the Poonjar Raja expanded his territory, acquiring lands from Peerumade to Devikulam, now part of Idukki District. In 1749-50, after the annexation of Thekkumkur and Vadakkumkur, the Poonjar Principality was incorporated into Travancore, and its history became aligned with Travancore's. The District’s participation in the freedom struggle was relatively limited compared to other parts of the State.

At the turn of the 20th century, Travancore State had four Revenue Divisions. Although not a formal revenue unit, Cardamom Hill was treated as a separate Division for magisterial, planning, and census purposes. In 1909, a fifth Division, Devikulam, was created by merging regions from adjoining Taluks of the Kottayam Division, and it was later renamed the High Range Division. There were no jurisdictional changes for the next decade. However, between 1931 and 1941, the High Range Division was merged with the Northern Division. No significant changes occurred until September 1956, when, on October 1, Udumbanchola Taluk was newly formed by taking two villages from Devikulam Taluk and one from Peerumade Taluk.

The history of the current population in the District is relatively recent, marked by colonization in challenging conditions involving harsh weather, wild animals, and epidemics. It also includes the exploitation of labor and labor struggles. Settling in the District began earnestly during the ministry of Sri. T.K. Narayana Pillai as part of the "grow-more food" campaign. These early settlers, including a mix of planters and other people, formed the foundation of the present population. Systematic colonization started during the ministry of Sri. Pattom Thanu Pillai, with the Kallar Pattom Colony in Udumbanchola Taluk bearing his name. The first human habitation in the District, which began in the early 20th century, is linked to a story involving Maharaja Sree Moolam Thirunal, who, while overseeing the construction of the Mullaperiyar Dam, was given milk by a shepherd named Ankur Rauther. Grateful, the Maharaja granted Rauther extensive forest lands, which were later sold to landowners in Tamil Nadu. These lands were transformed into cardamom and tea plantations using cheap labor. The Munnar area, developed over time as a British summer resort, became a Tamil enclave within Kerala due to its proximity to Tamil Nadu.