First granted by colonial governor Jonathan Belcher in 1735 as "Number 9" (ninth in a line of forts to guard against Indian attacks), it was regranted in 1753 as "Dupplin", after Scottish lord Thomas Hay, Viscount Dupplin, by Governor Benning Wentworth.

[1] The highest point in Lempster is the summit of Bean Mountain, in the eastern part of town at 2,326 feet (709 m) above sea level.

The western side of town is drained by the Cold River and its tributary, Dodge Brook, reaching the Connecticut in Walpole.

New Hampshire Route 10 traverses the town, connecting it with Goshen and Newport to the north and with Marlow, Gilsum, and Keene to the south.

The racial makeup of the town was 97.22% White, 0.10% Native American, 0.10% Asian, 0.10% Pacific Islander, and 2.47% from two or more races.

Lempster is home to New Hampshire's first wind farm, providing a total of 24 MW from 12 turbines, which started operation in 2008.