Born into a clerical family at his father's manse on 7 May 1825, he was the eldest son, in a family of 13 children, of Matthew Leishman, D.D., minister of Govan, leader of the middle party in the secession controversy of 1843; his mother was Jane Elizabeth Boog, and a brother, William Leisham, became professor of midwifery at Glasgow.
After the usual course at the Divinity Hall, he was licensed as a probationer by the presbytery of Glasgow on 7 February 1847, and became assistant minister at Greenock.
[1] In bad health, Leishman spent the winter of 1876–7 in Spain and Egypt, and investigated Mozarabic and Coptic service-books.
In 1892 he helped William Milligan to found the Scottish Church Society; he contributed papers to its conferences, and three times (1895-6, 1902–3, and 1905–6) acting as its president.
At Hoselaw, in a remote comer of the parish where Leishman used to conduct cottage services, a chapel was erected by public subscription to his memory in 1906.
In 1875, he published a plea for the observance by the Church of Scotland of the five major Christian festivals, entitled: May the Kirk keep Pasche and Yule?