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Harlington is a district of Hayes in the London Borough of Hillingdon and one of five historic parishes partly developed into London Heathrow Airport and associated businesses, the one most heavily developed being Harmondsworth. It is centred 13.6 miles (21.9 km) west of Charing Cross. The district adjoins Hayes to the north and shares a railway station with the larger district, which is its post town, on the Great Western Main Line. It is in the west of the county of Greater London and until 1965 it was in the south-west corner of the historic county of Middlesex.

The place-name Harlington is recorded in Anglo-Saxon as Hygereding tun: "Hygered's people's farmstead".

The earliest surviving mention of Harlington appears to be in a 9th-century charter in which land at Botwell in Hayes was said to be bounded on the west by "Hygeredington" and "Lullinges" tree. The first of these must be Harlington; the second has not been identified. The boundary between Hayes and Harlington, which may thus have been defined by the date of this charter, was later marked by North Hyde Road and Dawley Road, but Dawley Road may not have followed the boundary before the 18th century.

By 1834 the select vestry (informally known simply as the vestry) employed a paid assistant overseer. In 1824 a surgeon for the poor of Cranford and Harlington was appointed by the vestries of both. Their later co-operation saw the establishment of Harlington's National School jointly with in 1848, and its cottage hospital jointly with Cranford and Harmondsworth in 1884.