Millbourne is a borough in Delaware County, Pennsylvania, United States. The population was 1,159 at the 2010 census. Millbourne borders Philadelphia along Cobbs Creek. The borough is not part of neighboring Upper Darby, although Millbourne students attend Upper Darby School District schools. Magisterial courts with presiding jurisdiction for Millbourne are also located in Upper Darby.Millbourne, at over 16,000 people per square mile, is the most densely populated incorporated place in Pennsylvania, and 22nd in the entire United States.GeographyMillbourne Borough is located at (39.963412, -75.253998).According to the United States Census Bureau, the borough has a total area of 0.1 square miles (0.2 km2), all land, making it the smallest municipality in Delaware County, Pennsylvania by area.The borough is small enough that it is entirely within walking distance of its elevated station on the Market-Frankford Line. The Millbourne station, newly rebuilt and rededicated in mid-2008, offers an 11-minute commute to University of Pennsylvania and Drexel University and a 15-minute commute to Center City. Millbourne is also within easy walking distance of the 69th Street Terminal in Upper Darby and the 63rd Street Station in West Philadelphia, both of which are on the Market-Frankford Line. The Philadelphia Inquirer reported in the early 21st century that, according to U.S. Census data, only about one-third of Millbourne workers commuted by car. There is a PhillyCarShare shared car "node" at 69th Street Station, easily accessible from Millbourne or by commuters exiting the Market Frankford El at 69th Street.