The history of Ranni can be traced 5000 years back to the early settlement activity of Adi Dravidans around Sabarimala and Nilakkal. The temple at Sabarimala is an early temple of Kerala, dating back to around 2000 years. The deity's sitting posture and resemblance of the prayer 'Sharanam vili' to "Buddham Sharanam / Sangam Sharanam" points to the Buddhist past.[citation needed] Buddhist hermits always prefer to stay in interior jungles for solitary contemplation to attain Nirvana. The word 'Ayya' is a Buddhist Pali word means Lord/God. There was an ancient trade route passed through Nilackal between Kerala and Pandian kingdom.
Ranni Karthakal

The former rulers of Ranni was known as Ranni Kartha or Karthkal of Ranni they were the "Samanthas" and blood related family of Thekkamamkoor Raja.18th Century Anizham Thirunal Marthandavarma of Vennad annexed Thekkamkoor kingdom and confiscated all the lands belongs to the former rulers. Ranni Karthas Kioyikal (Palace) were stand at a place called Areekkal.Ranni karthas build Ranni Manjor Ramapuram temple.The temple is same as the construction of Aranmula Sreeparthasaradhy temple but the darshan is towards southern direction.

The Famous Devi Temple Thottamon Kavu Devi temple is its famous landmark. Thottamonkavu devi is believed the daughter of Paramashiva so the song "Mukkannan Thirumakale is famous" where Mukkannan is Shivan. It is a part of Ranni Taluk which lies in the valley of the Western Ghats.

Thottamon kavu Devi Temple is a resting place for pilgrims to sabarimala.The temple belongs to some nair families in Ranni. The family who administrated the temple were known as naaluveedanmaar, the four nobles. The four panikar families are who held the title were Meempallil, kerukattu(Padiyarakkalayil), Pangattu and cheruthalayathu .

In the 18th century, Knanaya Christians migrated to Ranni from Kallissery and later established the church, St. Thomas Church of Ranni. A significant number of Malankara Orthodox Christians & Mar Thomites also migrated from surrounding areas. In the early 19th century, Dr. Claudius Buchanan visited Ranni and was presented an old hand written Syriac Bible, from Ranni St. Thomas church.

Ranni was a part of Central Travancore region before the unification. Until the formation of Pathanamthitta district in 1982, Ranni was a part of Kollam district.

Ranni is largely dependent on private buses. Most of the long route KSRTC buses ply on the State Highway - 08. The State Highway - 08 (Punalur-Pathanamthitta-Pala-Muvattupuzha Road) connects the town to other major towns. There is a KSRTC bus depot in Ranni adjacent to Private bus stand. Auto rickshaws are available and generally hired for short distance (1–3 km) where bus service are non-existent or rare. Jeep is another preferred mode of transport where the terrain is hilly or rugged.

A train service connecting Ranni is one of the long-standing demands. Although the Union Government has cleared a proposal in 1995, the project awaits implementation. The nearest railway stations are at Thiruvalla and Chengannur which are at a distance of nearly 30 km each.

The nearest airports are Cochin International Airport (110 km) and Thiruvanathapuram International Airport (125 km).