He fought with distinction during the French Revolutionary and Napoleonic Wars, and later served as the last Governor of Galway and as Member of Parliament for Windsor.

Apprenticed to Mr. John Gelderd, a tannery owner of the village of Meanwood near Leeds, West Yorkshire, he became engaged to his master's daughter Anne.

I led on one Squadron to the Charge as a forlorn hope and out of 80 men I had not a dozen left – a very severe List of Killed and Wounded you will see by the Gazette.

In the charge he was knocked down, together with his horse, the fall breaking his leg; and although continually ridden over by friend and foe in the melee, Elley, nothing daunted, cheered on his men to fight for the honour of old England, and at last, catching hold of Sergeant Cooper's stirrup, was dragged to the rear.

[12] As a colonel in the 1st Regiment of Life Guards, Sir John was appointed Deputy Adjutant-General of Cavalry at the start the Waterloo Campaign.

[13][14] Two days after attending the Duchess of Richmond's Ball, he led the charge of the Life Guards during the holding action at Genappe during the Anglo-allied retreat from Quatre Bras to Waterloo.

[18] Sent to Ireland following the war, Elley was promoted to major-general in August 1819[19] and was made a Knight of the Royal Guelphic Order that same year.

[26] In 1836 the Eton & Windsor Gazette complained of the undue influence of the Castle on elections for that seat: It is well known in this Borough there are a number of Electors who hold situations in the Royal Establishment, but who reside in the town.

This it was that occasioned the return of Sir John Elley at the last election, who is frequently a guest at the Castle, and who of course obtains the influence of the many Tory hangers-on of the Court.

His tomb, in the north quire aisle, includes a bust in white marble and an inscription[28] that reads in part, "Unaided by dignity of birth, or the influence of fortune, he raised himself to the highest rank in the British Army by distinguished conduct in the field."

Though unmarried, Sir John was believed to have fathered at least one illegitimate son, who, legend has it, emigrated to Van Diemen's Land (Tasmania).

However, an Isabella Elizabeth Elley, daughter of the Reverend William Gifford Cookesley, of Eton College, was to receive a legacy of £300.