Canonsburg is a borough in Washington County, Pennsylvania, United States, 18 miles southwest of Pittsburgh. The population was 9,735 at the 2020 census. Canonsburg was laid out by Colonel John Canon in 1789 and incorporated in 1802. The town lies in a rich coal district, and most of the town's work force once worked in local steel mills or coal mines. Interstate 79 and U.S. Route 19 pass through the town, as does the Pittsburgh and Ohio Central Railroad.