Fitzwilliam was originally settled as Monadnock Number 4, one in a line of eight towns settled by Scottish colonists. Governor Wentworth named the town to honor his cousin William, fourth Earl of Fitzwilliam. Early grantees in Fitzwilliam included General James Reed and Matthew Thornton, signer of the Declaration of Independence. Fitzwilliam had one of the earliest granite quarrying operations in New Hampshire. It is the location of Rhododendron State Park, which has the largest rhododendron grove in New England and was designated a National Natural Landmark in 1982.
Population change for Fitzwilliam totaled 1,363 over 56 years, from 966 in 1960 to 2,329 in 2016. The largest decennial percent change was a 41 percent increase between 1960 and 1970, and a 32 percent increase the next decade. The 2016 Census estimate for Fitzwilliam was 2,329 residents, which ranked 128th among New Hampshire's incorporated cities and towns. The town has 67.3 persons per square mile of land area (info from NHES website)