| Core territory | East Lothian, Ayrshire, and the Scottish Borders |
| Gaelic form | Mac Raghnaill / Roinnidh |
| Notable figures | John Rennie Sr (engineer), John Rennie Jr (engineer) |
Rennie is a Scottish surname with two possible origins. The first is a diminutive of the given name Ranald or Reynold — a Norman personal name derived from the Germanic Raginwald, meaning "counsel-power." Families named Rennie (or Rainnie, Rainey, Ren) across Scotland are likely the descendants of men who bore this name in medieval times. The second possibility is that Rennie derives from the French place-name Reni or Renay, suggesting a Norman-origin family who came to Scotland in the 12th century.
Both origins converge on the same result: a family name well established in Lowland Scotland by the 16th century, concentrated in East Lothian, Ayrshire, and the Scottish Borders, and recorded in parish registers and burgh records from early in the period of civil registration.
John Rennie (1761–1821) was born in Phantassie, East Lothian, the son of a prosperous farmer, and became one of the greatest civil engineers of his age. He studied at Edinburgh University under John Robison and went to London in 1784, where he spent the rest of his working life reshaping the infrastructure of Britain.
Rennie's greatest works are in London. He designed Waterloo Bridge (opened 1817), considered by contemporaries one of the finest bridges ever built. He designed the new London Bridge (completed 1831, by his son after his death) — the bridge that was eventually purchased and reassembled in Lake Havasu City, Arizona, where it stands today. He also built Southwark Bridge, the Kennet and Avon Canal, Plymouth Breakwater, and numerous docks and harbours across Britain.
His son, Sir John Rennie (1794–1874), completed many of his father's projects and added his own, including the Royal William Victualling Yard in Plymouth, one of the great industrial buildings of the 19th century.
The Rennie family's trajectory — from East Lothian farming stock to the heights of British civil engineering within two generations — is a story that mirrors the broader arc of the Scottish Enlightenment: a society that produced extraordinary technical and intellectual achievement from relatively modest origins.
Beyond the Rennie engineers, the name appears throughout Ayrshire, the Scottish Borders, and Fife in parish records. The spelling varies — Rainnie, Renney, Ren — but all are likely variants of the same family name. In the United States and Canada, Rennie families appear in the migration records of the 18th and 19th centuries, particularly from East Lothian and Ayrshire.
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