Lord Uxbridge, whose family seat was at Beaudesert, lost his right leg when it was hit by a cannot shot, remarking to the Duke of Wellington: "By God, sir, I've lost my leg."
The letters were uncovered by Staffordshire County Council.
They are due to go on display.
The council said Lord Uxbridge was standing near the Duke of Wellington when he was hit in one of the last cannon shots of the battle.
In a famous exchange, he said to the duke: "By God, sir, I've lost my leg", to which Wellington is reputed to have replied "By God, sir, so you have".
Letters, eye-witness accounts, a charter praising Uxbridge's bravery and sufferings, a map of Napoleon's grave and a diary reporting the battle from the homefront are among the documents discovered by the council's archive team.
They also found an address from people in Burton-upon-Trent, sent to Lord Uxbridge, commending him on his bravery and a letter of response from him to similar praise from the people of Colwich.
Councillor Ben Adams said: "It's a fascinating collection of records that offers a valuable insight into the bloody but decisive battle."
The exhibition, which marks the 200th anniversary of the battle, will be touring Staffordshire libraries until October 2015.
Rare letters found in a council's archives have shed light on a Staffordshire army officer who famously lost his leg at the Battle of Waterloo, archivists say.