He was the fourth living son of Johann Ernest IV, Duke of Saxe-Coburg-Saalfeld, the third born by his father's second wife Charlotte Johanna of Waldeck-Wildungen.

When Christian Ernest married unequally in 1724, Francis Josias claimed the sole inheritance of the duchy.

In 1735, the support of the line of Saxe-Meiningen allowed him to effectively rule over Coburg on his own right, and Christian Ernest's death in 1745 made him the sole Duke.

From 1750 to 1755, he was regent of the duchy of Saxe-Weimar on behalf of Ernest Augustus II Konstantin.

In Rudolstadt on 2 January 1723 Franz Josias married Princess Anna Sophie of Schwarzburg-Rudolstadt.