The Gaelic name of the river is Leamhn, meaning the smooth stream, which anglicises to Leven (as Gaelic mh is spirantised). The surrounding area is the field of the smooth stream - Leamhnachd in Gaelic; this was originally anglicised as Levenauchen / Levenachs, then softened into Levenax / Lennax, and eventually the area was known simply as Lennox. Lomond is simply an alternative anglicisation for Leamhnachd, produced by assuming the m should be pronounced in the English manner.
Lennox was not one of the so-called seven ancient Provinces of Scotland, but formed as a province in the Middle Ages. The district embraced the whole of the ancient sheriffdom of Dumbarton: the parishes of Rosneath, Arrochar, Row, Luss, Cardross, Bonhill, Dumbarton, Kilmaronock, New Kilpatrick, Old Kilpatrick, Baldernock, Buchanan, Drymen, Killearn, Balfron, Fintry, and Strathblane, with Campsie and Kilsyth, being all within the bounds ruled over by the Earls of Lennox.
Under local government reforms in the mid 19th century, the province of Lennox was re-structured as the County of Dunbartonshire, when the north-eastern shore of loch Lomond was transferred to Stirlingshire.