Darby is a borough in Delaware County, Pennsylvania, United States, along Darby Creek 5 miles (8.0 km) southwest of downtown Philadelphia. It has a public library founded in 1743 and a cemetery more than 300 years old. The Quakers lived there early in the colonial era. Darby was settled about 1654, and was incorporated on March 3, 1852. In 1900, 3,429 people made their homes there, in 1910, 6,305, and in 1940, 10,334 residents of Darby existed. The population was 10,687 at the 2010 census. There is speculation, according to local historians, that the name "Darby" is related to "Derby" derived from the English town of Derby, in the county of Derbyshire. One explanation states that the Darby Creek closely resembled the English River Derwent, the main body of water that flows through Derby. British immigrants named the town in the new world thus.
Birthplace of comedian and actor, W.C. Fields, born in 1880 at the Arlington Hotel, then located at 832 Main Street.
Darby is home to the 5th oldest all volunteer Fire Department and 2nd oldest Free Library in the United States.
Darby Borough is a distinct municipality from the similarly named, nearby municipality of Darby Township.