The Town of Hadley is in the northern part of the county and is west of Glens Falls.
The first settler arrived around 1788.
The town was formed from parts of the Towns of Greenfield and Northumberland in 1801. The Town of Corinth was taken from Hadley in 1818, and the Town of Day was removed in 1819, bringing the Town of Hadley to its current dimensions.
In 1930, the Conklingville Dam by the west town line helped create the Sacandaga Reservoir and controlled flooding.