Forrest was the sixth child of William and Margaret Forrest. He was born at his parents' flour mill on Preston River at Picton, near Bunbury, Western Australia. He initially attended a one-room school that his father had built on his property, then later attended Bishop Hale's school (now Hale School) in Perth.[1][2]
From 1873 to 1876, Forrest worked with sheep at Cubbine station near Quairading.[1][2]
In 1874,[2] Forrest drove a mob of sheep to a new lease of land (granted to Forrest, his brothers John and Alexander, and Septimus Burt) known as Minderoo, in the Ashburton district, and subsequently became the manager of the new station.
In 1881, Forrest married Mary Parker at York,[2] after a seven-year engagement.[1] They had seven children, three of whom died as infants. One of his sons, Mervyn Forrest, took over the family's pastoral interests, and like his father became a member of parliament.
