Sir John Leslie. 2nd Baronet of Glaslough

Leslie was the only son of Sir John Leslie, 1st Baronet, and Lady Constance Wilhelmina Frances Dawson-Damer. His father was the Conservative Member of Parliament for Monaghan from 1871 to 1880 and was created a baronet in 1876, and his mother was the daughter of Colonel George Lionel Dawson-Damer.

On 2 October 1884 in New York City, with disapproval from both families, Leslie married Leonie Blanche Jerome, daughter of Leonard Jerome and Clarissa Hall. Leonie's sister was Jennie, wife of Lord Randolph Churchill and mother of Winston Churchill. Leslie and his wife had five children:

Leonie Clarissa Leslie (1884 - 1945), married with the count Gregório de Almeida Sir John Randolph Leslie, 3rd Baronet (1885–1971), generally known as Shane Leslie Captain Norman Jerome Beauchamp Leslie (1886–1914) Seymour William Leslie (1889–1979) Lionel Alistair David Leslie (1900–1987) In his second autobiographical book, Lionel recounted various anecdotes about his immediate family and their home at Castle Leslie in County Monaghan.

Career and later life Leslie was commissioned into the Grenadier Guards, with whom he served as a lieutenant at the Battle of Tel el Kebir in 1882, distinguishing himself under fire. He was appointed lieutenant-colonel of the 5th Battalion, Royal Irish Fusiliers in April 1902, and later became honorary colonel of the battalion.

He served as High Sheriff of Monaghan in 1905 and was also a Justice of the Peace and Deputy Lieutenant for the county.

The Leslies were opposed to the Home Rule movement. Leslie led the Monaghan Militia in the 1890s and he allowed the Ulster Volunteer Force drill at the demesne in 1914. The Leslie family were one of the largest land-owning families in the late 19th century. Their holdings comprised 70,000 acres in counties Cavan, Donegal, Down, Fermanagh, Meath and Tyrone.