Windham County was created from Hartford and New London counties on May 12, 1726, by an act of the Connecticut General Court. The act establishing the county states:
That the west bounds of the town of Lebanon, the north
bounds of Coventry, the north bounds of Mansfield till it
meet with the southwest bounds of Ashford, the west bounds
of Ashford, the east bounds of Stafford, the Massachusetts
line on the north, and Rhode Island line on the east, the north
bounds of Preston, and north bounds of Norwich, containing
the towns of Windham, Lebanon, Plainfield, Canterbury,
Mansfield, Coventry, Pomfrett, Killingly, Ashford, Voluntown
and Mortlake, shall be one entire county, and called by the
name of County of Windham.
In May 1749, the town of Woodstock, formerly New Roxbury, Worcester County Massachusetts, was unilaterally annexed by Connecticut and assigned to Windham County. In 1785, the town of Union (incorporated in 1734) was transferred to the newly-formed Tolland County. Over the next century, Windham County would lose several towns to Tolland and New London counties: Coventry to Tolland in 1786, Lebanon to New London in 1824, Columbia and Mansfield to Tolland in 1827, and Voluntown to New London in 1881. The final boundary adjustment occurred on April 7, 1885, when the boundary dispute between the towns of Windham and Mansfield was resolved.