In 1885 the Prussian government established districts within the province of Hanover. The present territory of the district was occupied by four districts: Hildesheim, Alfeld, Gronau and Marienburg. In 1932 the district of Gronau joined Alfeld, the number of districts was reduced to three.
When the state of Lower Saxony was founded in 1946, the districts were reorganised: Hildesheim became a district-free city, the remaining district of Hildesheim and Marienburg were merged to the new district of Hildesheim-Marienburg. The district of Alfeld remained in its former borders.
In 1974 the city of Hildesheim lost its status as a district-free city and became part of the surrounding district, which was renamed to Hildesheim. The districts of Hildesheim and Alfeld were merged in 1977.
The district is located southeast of Hanover, and the northernmost parts are bordering the Hanover metropolitan area. The northern half of the district is highly urbanised, while the south is covered with forests. The Leine river enters the district in the south, runs through the towns of Alfeld and Gronau and leaves in the northwest towards Hanover; a small affluent, the Innerste, joins the Leine from the east after flowing through the city of Hildesheim. Between Leine and Innerste there are the forested hill chains of the Hildesheimer Wald ("Hildesheim Forest") and the Sieben Berge ("Seven Mountains", see Alfeld for more details).