It is believed that the Town of North Castle was originally inhabited by the Siwanoy, part of the Wappinger Confederacy and members of the Algonquin nation. The Siwanoy were taken by force in 1644 by Europeans. In the early 1700s, King William gave his favorite courtiers the West Patent, of which the western portion of North Castle was a part, and the Middle Patent, the eastern part of North Castle. At one time, North Castle included all the territory that became incorporated as New Castle in 1791. The territory comprising both towns was once part of the Parish of Rye organized in 1693.

The area quickly became a refuge for people fleeing from religious persecution. People from Massachusetts and Connecticut settled the eastern part of North Castle, while Quakers from Rye and Long Island flocked to Armonk. By 1730, North Castle was an established settlement.

The Town of North Castle was incorporated on March 7, 1788. North Castle's name is said to derive from a barrier built by the Mohican people to protect themselves from enemy attacks which stood on the hillside now occupied by the international headquarters of I.B.M. Corporation. They called the site "North Fort" and European settlers later gave it the name of North Castle. The name Armonk is derived from another Mohican word, 'Cohamoog', which means 'the wide, flat place where the water runs'.

During the American Revolution, New and North Castle were officially considered neutral territory. However, the area was strongly patriotic. One significant Revolutionary War conflict did occur in North Castle, "The Battle of White Plains". This battle of October 28, 1776 was a series of short skirmishes between General George Washington's small American army and General William Howe's much larger British & Hessian force. Although the British eventually won the confrontation, forcing Washington's troops to retreat, Howe never followed up this advantage by pursuing and capturing the American army. Thus, the battle served as a delaying action that allowed Washington's troops to withdraw to safety in New Jersey. As a result, many historians feel that the battle marked an important turning point in the war.

During the Revolutionary War, the Elijah Miller house in North White Plains served, several times, as the headquarters of General George Washington. A few miles to the west (now known as Mount Kisco) St. George's Church (North Castle Church) served as a camp and hospital. A young Frenchman's diary dated July 6, 1781, reported of the area:
"This whole country gives evidence of the horrors of war... All the Whigs here have abandoned their houses. Among them are some very handsome ones, deserted, half destroyed, or burned, with untended orchards and gardens filled with fruits and vegetables and driveways overgrown with grass two feet high."

Point of Interest: Press here for "People of the The American Revolution"

Point of Interest: Smith's Tavern in Armonk is believed to have been built in the late 1700's. John Smith, a former captain in the Continental Army, operated the house as a tavern, site of town meetings, colonial militia headquarters, post office, and stopping place for the Danbury stage as early as 1797. Smith's son Samuel continued to operate the tavern until his own death in 1884. Since 1974, the building has belonged to the North Castle Historical Society and is now open to the public as a museum.

During the early part of the 19th century, most North Castle residents were farmers. However, several small "cottage industries" did exist. For example, some farmers supplemented their income by becoming shoemakers or shirtmakers. The coming of the railroad in the 1840's marked the beginning of the shift away from the region's agricultural way of life. However, towns without the railroad, such as North Castle, suffered economically. North Castle was also hurt economically by the Industrial Revolution since new manufacturing techniques made the local cottage industries impractical. From 1860 to 1900, North Castle's population declined from 2,200 to 1,470.

Point of Interest: The Underground Railroad, which helped runaway slaves travel to freedom in Canada, operated a "station" between Armonk and North White Plains.

By the early 20th century, North Castle's economy improved dramatically due to the New York City purchase of reservoir land and the building of the Kensico Dam in Valhalla (1909-1915) which used North Castle granite. Many of the European immigrant stone masons who built the dam later settled in the town's Quarry Heights section.