Fairfield County was the home of many Native American tribes prior to the coming of the Europeans. People of the Schaghticoke tribe lived in the area of present-day New Fairfield and Sherman. From east to west the Wappinger sachemships included the Paugussetts, Tankiteke, and the Siwanoy. There were also Paquioque and Potatuck inhabitants of Fairfield County. The Dutch explorer Adriaen Block explored coastal Connecticut in the Spring and early Summer of 1614 in the North American built vessel Onrust. The first European settlers of the county, however, were Puritans and Congregationalists from England. Roger Ludlow (1590–1664), one of the founders of the Colony of Connecticut, helped to purchase and charter the towns of Fairfield (1639) and Norwalk (purchased 1640, chartered as a town in 1651). Ludlow is credited as having chosen the name Fairfield. Fairfield is a descriptive name referring to the beauty of its fields. The town of Stratford was settled in 1639 as well by Adam Blakeman (1596–1665). William Beardsley (1605–1661) was also one of the first settlers of Stratford in 1639.
Fairfield County was established by an act of the Connecticut General Court in Hartford along with Hartford County, New Haven County, and New London County; which were the first four Connecticut counties, on May 10, 1666. From transcriptions of the Connecticut Colonial Records for that day:
- This Court orders that from the east bounds of Stratford
- to ye bounds of Rye shalbe for future one County wch
- shalbe called the County of Fairfield. And it is ordered
- that the County Court shalbe held at Fairfield on the second
- Tuesday in March and the first Tuesday of November
- yearely. (sic)
The original Fairfield County consisted of the towns of Rye, Greenwich, Stamford, Norwalk, Fairfield, and Stratford. In 1673, the town of Woodbury was incorporated and added to Fairfield County. In 1683, New York and Connecticut reached a final agreement regarding their common border. This resulted in the cession of the town of Rye and all claims to the Oblong to New York. From the late 17th to early 18th centuries, several new towns were incorporated in western Connecticut and added to Fairfield County, namely Danbury (1687), Ridgefield (1709), Newtown (1711), and New Fairfield (1740). In 1751, Litchfield County was constituted, taking over the town of Woodbury. The final boundary adjustment to Fairfield County occurred in 1788 when the town of Brookfield was incorporated from parts of Newtown, Danbury, and New Milford, with Fairfield County gaining territory from Litchfield County.
Other early county inhabitants include:
During the Revolutionary War, Connecticut's prodigious agricultural output led to it being known informally as "the Provisions State". In the spring of 1777, the British Commander-in-Chief, North America General William Howe, in New York City, ordered William Tryon to interrupt the flow of supplies from Connecticut that were reaching the Continental Army. Tryon and Henry Duncan led a fleet of 26 ships carrying 2,000 men to Westport's Compo Beach to raid Continental Army supply depots in Danbury on April 22, 1777. American Major General David Wooster (1710–1777), who was born in Stratford, was in charge of the stores at Danbury and defended them with a force of only 700 troops. Sybil Ludington helped rally New York militia to aid in the defense of Danbury. The New York militia included Sybil's father Colonel Henry Ludington. Though they arrived too late to save Danbury from burning, the elder Ludington and the New York militia helped support the Danbury troops and ensuing engagement of the British known as the Battle of Ridgefield on April 27, 1777. Wooster was wounded at Ridgefield and died five days later in Danbury.
Two years later during a British raid on Greenwich on February 26, 1779 General Israel Putnam, who had stayed at Knapp's Tavern the previous night, rode away on his horse to warn the people of Stamford. Putnam was shot at by the British raiders but was able to escape. The hat he was wearing with a musket ball hole in it is on display at Knapp's Tavern in Greenwich (which is commonly, albeit somewhat erroneously, called Putnam's cottage). In the summer of 1779, General William Tryon sought to punish Americans by attacking civilian targets in coastal Connecticut with a force of about 2,600 British troops. New Haven was raided on July 5, Fairfield was raided on the 7th and burned. Norwalk was raided on July 10 and burned on the 11th. Norwalk militia leader Captain Stephen Betts put up resistance to the invaders, but was overwhelmed by the powerful British raiders and was forced to retreat.
David Sherman Boardman (1786–1864) was a prominent early lawyer and judge in this and neighboring Litchfield County.
On October 7, 1801, Neremiah Dodge and other members of the Danbury Baptist Association wrote a letter to then-president Thomas Jefferson expressing their concern that as Baptists they may not be able to express full religious liberty in the state of Connecticut whose "ancient charter" was adopted before the establishment of a Baptist church in the state. Jefferson replied in a letter to Dodge and the other members of the Danbury church on January 1, 1802, in which he stated that the First Amendment to the United States Constitution provided "a wall of separation between church and State" that protected them.
Although it is often viewed as an extension of metro-New York City, Fairfield County has had much industry in its own right. Bridgeport Machines, Inc., a milling machine manufacturer, was founded in Bridgeport in 1938. Stamford, Connecticut is an example of edge city urbanization, with many large and important companies having offices there and benefitting from proximity to New York.
At the height of its influence in the 1920s, the Ku Klux Klan had a distinct presence in the county and county politics. The group was most active in Darien. The Klan has since disappeared from the county.
Fairfield County, along with all other Connecticut counties, was abolished as a governmental agency in accord with state legislation that took effect October 1, 1960.