Sir John Leslie, 1st Baronet, JP, DL (16 December 1822 – 23 January 1916) was a British Conservative politician, landowner and painter. He was Member of Parliament for Monaghan between 1871 and 1880. He succeeded his elder brother, Charles Powell Leslie, in that role.^[1]

He was the second son of Charles Powell Leslie and his second wife, the former Christiana Fosbery (died 1869). Among his siblings was Christiana, the Marchioness of Waterford.^[2] He was educated at Harrow and Christ Church, Oxford. He obtained a commission as a Captain in the 1st Life Guards. However, after embarking on a Grand Tour in 1847 he resigned his Army commission and pursued artistic interests.^[1]

He was a Justice of the peace and Deputy lieutenant for counties Monaghan and Donegal.^[3] In 1876, at the recommendation of Disraeli, he was made 1st Baronet Leslie, of Glaslough. He built Glaslough House as a replacement for Castle Leslie.^[1]

In 1854, he met at Hazelwood a distinguished Waterloo veteran, Colonel George Dawson-Damer (younger brother of the 2nd Earl of Portarlington), and his daughter, Lady Constance, whom Leslie married two years later at St George's, Hanover Square, London.^[2]

He owned acres 44,000 acres, with 14,000 in Monaghan, 29,000 in Donegal and 1000 in Tyrone.^[4]

Sir John Leslie married Lady Constance Dawson-Damer, sister of the 4th Earl of Portarlington, and had one son and four daughters:

• Sir John Leslie, 2nd Baronet (1857–1944)^[2]
• Mary Leslie (1858–1936)^[2]
• Constance Christina Leslie (1861–1945)^[2]
• Theodosia Leslie (1865–1940)^[2]
• Olive Louisa Blanche Leslie (1872–1945)^[2]