The first inhabitants of Mainit settled near Lake Mainit in what is now barangay San Isidro. It was believed that the constant harassment of pirates forced the original settlers to move to the present town site.
Mainit was made into a barrio in 1904 under the Municipality of Placer. In 1906, it became a Municipality of Agusan Province but after six months, Mainit was returned to the Province of Surigao del Norte and its status was reverted to being a barangay of Placer. Mainit was finally made into a municipality in Jan. 1, 1931 by virtue of Executive Order 290 dated December 27, 1930 signed by Governor-General Dwight F. Davis.
Two of Mainit’s barangay was made into a Municipality: Tubod in 1958 and Alegria in 1968.
Mainit got its name from the hot sulfuric spring which flows to the river the “Mapaso Hot Spring”. Mapaso literally is “hot”.