Kokrajhar was originally a part of the undivided Goalpara district.In 1957 it was craved out as a Civil Sub-division from the then Dhubri Sub-division of Goalpara district. Till then it was merely small town with a railway station. On 1st July, 1983 it was upgraded into a district. At that time, there were four Police stations viz. Bijni, Sidli, Kokrajhar and Gossaigaon with a total area of 4,065.88 Sq.Km. spreading from the river Manas in the east to the river Sonkosh in the west. In 1989, the district of Bongaigaon was curved out with about 40% area of Kokrajhar district. Later on, the Nayekgaon G.P. of Dhubri district with an area of 40.22 Sq.Km. was merged with this district. Again on 10th February, 2009 Chirang district was created by curving out parts of Kokrajhar district and some parts of Dhubri district were merged with Kokrajhar district. The present area of the district is estimated to be 3,169.22 Sq.Km.
The vegetaion of the district is characterized mainly by lush green forest and varieties of flora and fauna. Manas National Park stands out as the glaring example of this bio-diversified feature of the district.
The climate of the district is sub-tropical in nature with warm and humid summer and also followed by cool and dry winter.