Thomas Robertson (II) was the youngest of five children of Thomas Robertson (1785-1872) and his wife Anne (née Lachlan). The other children were Jane (Jean) (1816-1895); James (1818-1893); Margaret (1820-1909); and John (1822-1905). Each of them married and had 7, 10, 9, and 7 children respectively. Eight of these thirty-eight died before they reached the age of 20.
In 1839 the Robertsons emigrated to Australia and arrived at Port Phillip, the large bay on which Melbourne and Geelong are situated, on board the
John Bull
on January 21st 1840. The elder Thomas took with him his children (his son Thomas was then fifteen) and the six children of his sister Mary Gordon and her husband, both dead by that time; the youngest of their children was 11 years old. The Robertson family had been Presbyterians in Scotland, and when they got to Australia they seem to have married other Scots, probably also Presbyterians.