[1] He first showed at the Royal Academy in London in 1820, making his debut with a portrait of Mrs. Yates in the role of Meg Merrilies.

Prince Augustus Frederick, Duke of Sussex and the Marquess of Bristol sat to him, as did various prominent Irishmen including the Earl of Rosse, Richard Lalor Sheil MP, Sir Matthew Tierney M.D.

[1] His subject pictures included The Battle of the Nile and The Battle of Trafalgar (1825); Mary Stuart's Farewell to France, (1826, engraved); Jacques and the Wounded Stag (1830); Escape of Mary Queen of Scots from Loch Leven Castle, (1837, painted for Lord Egremont) and Lady Jane Grey summoned to Execution (1844).

[5] Despite his move to England, Leahy continued to exhibit in Dublin occasionally until 1846, sometimes showing works that had been seen in London some years earlier.

When his Mary Queen of Scots' Farewell to France was shown in 1842 it was purchased by the Royal Irish Art Union.

[1] During a visit to Cork in 1846, Leahy painted a portrait of Father Mathew, the "Apostle of Temperance",[3] in connection with a commission from Dwarkanath Tagore.