In 1837 by Act of Parliament the new District of Wellington was organized and a court house and jail in the town of Guelph were authorized.

The District Council of the County of Wellington was formed and consisted of eighteen municipalities. One of them, which opened in 1835 with area of 261 km², was named after Arthur Wellesley, the First Duke of Wellington.

The first Wellington County Council meeting was held on January 23, 1854. The County developed into an individual entity consisting of the Townships and Towns of Amaranth, Arthur, Eramosa, Erin, Guelph, Guelph (Town), Garafraxa, Maryborough, Nichol, Peel, Pilkington, and Puslinch.

The City of Guelph separated in 1879 and became incorporated as a City. Luther Township was divided into East and West. Orangeville and Garafraxa East linked Dufferin County and were no longer represented on Wellington County Council.
Merger in 1999 resulted in the formation of seven new municipalities as follows: Township of Centre Wellington, Town of Erin, Township of Guelph/Eramosa, Township of Mapleton, Town of Minto, Township of Puslinch, and Township of Wellington North.