John Kerr Clark was born in Glasgow, Scotland, in 1838, the only child of James Clark and his first wife Margaret, nee Kerr (d. 1838). After his education in Scotland he spent some years travelling in Europe, before leaving for Australia in 1860 with his uncle Duncan Clark (b. 1829) to set up a sheep station at Ghoolendaadi, near Boggabri, in New South Wales. In 1870 he married Kate Louisa Robertson (1846-1926), the daughter of neighbouring landowner and politician Sir John Struan Robertson (1816-1891). One of nine siblings, she had grown up in at the Robertson's home, Clovelly, in Sydney, where she also spent a lot of time after she married to avoid the harsh living conditions at Ghoolendaadi. Between 1871 and 1883 John and Kate Louisa had eleven children, of which four, three boys and a girl, died in infancy. The family moved to England in 1889, first to Bath, later to London. John Kerr Clark returned to Ghoolendaadi in the late 1890s and died in Australia in 1910.

James Clark was born at Blairmore, near Inverchapel (then Inverchapple) in Argyll, Scotland, in 1813, the eldest son of the farmer Robert Clark. He trained as a surgeon at the University of Glasgow, but never practised and instead pursued a business career, first with a firm of Greenock merchants, Kerrs & McBride, then with the cotton manufacturing company James Finlay & Co. By the 1850s, his business success had allowed him to buy the sizeable Crossbasket estate near Glasgow and become a prosperous member of the landed gentry. He married his second wife, Agnes, nee Barclay (1824-1863), in the mid-1840s, and had seven children with her. After James Clark's death in 1876, a family dispute broke out between John Kerr Clark in Australia and his stepsiblings in Scotland, which would last for decades, and led to John Kerr Clark's family adopting the name 'Kerr' as part of the surname to distinguish themselves from the 'Barclay' Clarks.