Friedrich Josias was born at Callenberg Castle, the third son and youngest child of Charles Edward, Duke of Saxe-Coburg and Gotha, and Princess Victoria Adelaide of Schleswig-Holstein-Sonderburg-Glücksburg.
He was stationed in June 1944 in Denmark under General von Hanneken, where he was captured by the British in May 1945 but released in autumn 1945.
The third of three sons, Friedrich Josias was not expected to succeed as head of the ducal family.
The couple had one son: In San Francisco on 14 February 1948, Friedrich Josias married secondly, Denyse Henrietta de Muralt (14 December 1923 – 25 April 2005).
They had three children: In Hamburg on 30 October 1964, Friedrich Josias married his third wife, Katrin Bremme (22 April 1940 – 13 July 2011).