In the history of the district it is mentioned that it was a part of the earlier Mithila region. The place of Mithila for the first time came into the lime light after being settled by Indo-aryan peoples who established the Mithila Kingdom as well. During the late Vedic period (C. 1100-500 BCE.) Mithila or Videha emerged as one of the major political and cultural centers of South Asia, along with Kuru and Pancala. The Videha Kingdom was later merged into the Vajji confederacy, which had its capital in the city of Vaishali. Purnia has a rich history of Mughal rule. When there was Mughal rule in this region at that time it was an outlying military province so it spent most of its revenue earned on the protecting of the borders and armies against tribes from the north and east. After the seizure of Calcutta in the year 1757, a skirmish of local governor was started against Siraj ud-Daulah. In 1765, the district came completely under the British possession, along with the rest of Bengal. Purnea district’s modern history was started during the time of East India Company on February 10,1770. Three districts namely Katihar in 1976, Araria and Kishanganj in 1990 were partitioned off from the territory of Purnia district. Legends believe that the district came into existence in the year 1912. Natives of the district believe that once upon a time Purnea region was Purna-aranya meaning "complete jungle" and that’s why it has got the name Purnea.

Purnia is part of the Mithila region. Mithila first gained prominence after being settled by Indo-aryan peoples who established the Mithila Kingdom (also called Kingdom of the Videhas). During the late Vedic period (C. 1100-500 BCE), Videha became one of the major political and cultural centers of South Asia, along with Kuru and Pancala. The kings of the Videha Kingdom were called Janakas. The Videha Kingdom was later incorporated into the Vajji confederacy, which had its capital in the city of Vaishali, which is also in Mithila.

Balai Chand Mukhopadhyay, a distinguished Bengali author, playwright, poet, and physician who wrote under the pen name of Banaphul meaning "the wild flower" in Bengali was born on 19^th July, 1899 at Manihari village in the district. He wrote thousands of famous novels and short stories. In his honour the Government of India issued a postage stamp featuring his image on the 100^th anniversary of his birth.

Geographically, the district lies at 25°78'N latitude and 87°47'E longitude and 536 m Altitude. In the year 2021, there was a total 1.72% forest area of total geographical area. The district encompasses a geographical area of 3,229 sq. kms. and it is bounded by Araria district on the north, Katihar district and Bhagalpur district on the south, West Bengal on the east and Madhepura district on the west. The climate of the district remains pleasant throughout year. Most of its rainfall occurs in the monsoon season. The actual rainfall in the district was 1446.8 mm in the year of 2021-22.

Administration wise, the district is divided into four sub-divisions including Purnea, Banmankhi, Baisi and Dhamdaha. There are 1 Lok Sabha seat and 7 Assembly constituencies in the district. Moreover, the district comprises 3 towns and 1,273 villages. The administrative language in the district is Hindi.

According to 2011 census the district has a population of 32,64,619 out of which 16,99,370 are males and 15,65,249 are females. The district has a sex ratio of 940 females for every 1000 males. During the year 2001-2011 the population growth rate in the district was 28.33% including 27.92% were males and 28.77% were females. As per 2011 census the major religions in the district are Hindu and Muslim with 60.94% and 38.46% of the total population. The population density in the district is 1,011 persons per sq. km. According to 2011 census the principal languages in the district are Hindi, Urdu and Maithili with 62.19%, 18.62% and 10.72%. In the year 2020 the number of live births in the district was 1,25,147 out of which 65,274 were males and 59,861 were females. In the same year the number of deaths in the district was 12,296 out of which 7,804 were males and 4,491 were females.

Most of the population in the district are engaged in agriculture to earn their livelihood. Every year a huge chunk of revenue comes from the agricultural products grows in the district. Some of its chief agricultural products are jute, tobacco, rice, barley, sugarcane, khesari, potatoes, mung beans, etc. Industrially, it is not so develop only jute mills are available in the district. In 2006 the Ministry of Panchayati Raj declared Purnia as one of the country's 250 most backward districts and currently receiving funds from the Backward Regions Grant Fund Programme (BRGF). In the year 2011-12 the gross domestic product in the district was Rs. 5,33,347 lakhs at current price and Rs. 3,19,568 lakhs at constant prices in the year 2004-2005. In the year 2011-12 the net domestic product in the district was Rs. 4,84,317 lakhs at current price and Rs. 2,86,299 lakhs at constant prices in the year 2004-2005.

In the whole Bihar region, the district has been renowned as a centre for education. Jawahar Navodaya Vidyalaya, Kendriya Vidyalaya, Vidya Vihar Residential School, etc. are some reputed school of the district. The medium of instruction in these schools are mainly English and Hindi. The district has also many colleges which imparts education on various professional courses likewise engineering, law fine art home science, etc. As per 2011 census the literacy rate in the district is 51.08% including 59.06% are males and 42.34% are females. The number of literates in the district is 13,32,326 including 8,05,261 are males and 5,27,065 are females.

Ajit Sarkar, an illustrious Indian politician and a member of the Bihar Legislative Assembly for four times from 1980 to 1998 from Purnia constituency, was born in the year 1947 in Purnia district. He was a member of Communist Party of India (Marxist).

Every year numerous travellers come to this place to enjoy its tourist places. Maa Kali Mandir, one of the most popular temples in this region. People from far distance place comes this temple to offer their prayers. Purani Devi temple is situated at a distance of 3 km. from the Purnia city. Natives of the district believe that the Purnia city got its name from the Purani Devi temple. Some of its other wonderful centres of attraction are Sarvodaya Ashram, Maa Rani Sati Mandir, Peer Baba ki Mazar (Bhawanipur), Maa Bhuwan Devi Mandir (Bhawanipur Rajdham), Kali Asthaan, Kajha Kothi, and Jalalgardh Fort, etc.