The Halls auction house in Shrewsbury says the two-sided letter, written by the author of one of the first dictionaries of the England language, was penned in March 1783.
It says the price paid, £13,200 plus buyer's premium, is a testament to the enduring legacy of one of English literature's greatest figures.
The Shropshire vendor bought the letter at Sotheby's around 1966 to prevent it from leaving the country at a time when Johnson memorabilia was being snapped up by American collectors, the late Donald F. Hyde and his wife Mary.
The couple were passionate collectors of Johnson material for more than six decades. Their collection, which contains no less than 746 Johnson letters - over half of the known total - was bequeathed to the Houghton Library at Harvard University following Mary's death in 2003.
Gerry Berwyn-Jones, the head of fine art at Halls, said: "As a result of the Hyde's dedication to collecting Johnson's letters, these items are now quite scarce on the market.
"It is a testament to the significance of Samuel Johnson that this letter signed by him has sold for such a considerable sum. The buyer of this extraordinary letter resides outside London and so the letter, in the short term at least, will stay in England and remain a part of Britain's cultural heritage.
"The letter is a glimpse into the mind of a man who made a lasting impact on the way we speak and write today. We are all delighted to have facilitated the sale of this exceptional piece of history and look forward to offering more unique items for sale in the future."
For further information on this auction or other upcoming sales at Halls, telephone 01743 450 700 or email firstname.lastname@example.org.