The land that became Norristown was bought by Isaac Norris and William Trent on October 7, 1704, for 50 cents an acre. In 1712 Norris acquired Trent's share and built a gristmill at the foot of present Water Street, and the town took the Norris family's name. Norristown was named the county seat when Montgomery County was formed in 1784, was incorporated as a borough in 1812 with about 500 people, and was enlarged in 1853.
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The borough grew rapidly after the Civil War, reaching 22,265 people by 1900 and 38,181 by 1940, when it was the most populous borough in Pennsylvania and was described as the most populous independent borough in the United States. At its height it was an industrial, retail, banking and government center, with breweries, cigar factories, textile mills, icehouses, foundries, rolling mills and lumberyards, two department stores and several theaters downtown. It was settled mainly by the English, with some Germans, Scots, Dutch and Swedes, and large numbers of Irish arrived in the mid-1800s, followed by Italians around the turn of the 20th century.
After World War II, new shopping malls in nearby King of Prussia and Plymouth Meeting drew business away, and the downtown declined. Industry followed as many companies closed or moved to new industrial parks elsewhere in Montgomery County.
Sources
Wikipedia: Norristown, Pennsylvania