Josiah Unthank was born the 28th of 2d mo 1780 in Guilford County, North Carolina, to Joseph and Judith (Thornburg) Unthank. This was Joseph Unthank's second marriage. He was 76 at the time of his son's birth, while his wife was 23. Joseph died when Josiah was only 2 months old. Joseph's two surviving sons from his first marriage were old enough to be Josiah's father themselves, and they had already been provided for by their father, so the bulk of Joseph's estate was left in trust for his baby son. Joseph also left for Josiah a written account of his own boyhood, growing up in Yorkshire, England. This account is now in the archives of Guilford College in North Carolina. Three years after Joseph's death Josiah's mother Judith married Jeremiah Horn and moved to Tennessee, leaving little Josiah in the care of his older half brother, Allen Unthank. Allen raised him with his own children, so Josiah grew up with his own nieces and nephews, some of whom were older than he.

In 1801, Josiah was disowned for marriage out of unity with Friends. He had married Anna Brittain (1779-1875), who would later become recorded as a minister in the Society of Friends. Josiah was readmitted to the Society of Friends in 1807 when he condemned his actions. In 1827 Josiah, Anna, and their children were received in membership at Milford Friends Meeting in Wayne County, Indiana, but appear to have returned to New Garden Meeting in North Carolina the following year. In 1830 Josiah and his family were granted a certificate to transfer from New Garden Meeting in NC to New Garden Meeting in Indiana.

Josiah's death occurred 6th mo 30 1833 in Wayne County, Indiana. His widow remarried in 1839 to William Hobbs and settled in Spiceland, Henry County, Indiana.