The county was created in 1564, and took its name from the principal town, properly known as Longfort Uí Fearghail, "O'Farrell's landing place". Known in earlier times as Annaly, the association of the area with the O'Farrells goes back to the ninth century. Despite the grant of the territory to the de Lacys in the twelfth century, the O'Farrells ruled until the beginning of the seventeenth century, when some plantation of English settlers took place. The presence of the Edgeworth family, one of whose members was the novelist Maria Edgeworth, dates from this period. Their name survives in the village they founded, Edgeworthstown.
Physically, the county is flat, rising slowly from the river Shannon which forms its western border, with many bogs and lakes.
Longford lost almost one-third of its people to starvation and emigration between 1841 and 1851, the years of the Great Famine. And the loss continued in subsequent emigration: by 1986 the population was only a quarter of what it had been in 1841.
Surnames associated with the county include Brady, Farrell, O'Reilly, O'Rourke. Kiernan, and Gaynor.