He was the youngest of the twenty children of Johann Adolf I, Duke of
Saxe-Weissenfels and Johanna Magdalena of Saxe -Altenburg.
On May 9th. 1721 Johann Adolf married Johannette Antoinette Juliane
of Saxe-Eisenach. They would have one son together. She died in 1726.
During the War of the Polish Succession, Johan Adolf led the Saxon troops
into Poland (1733). This army stayed there for the next three years till the
coronation of King Frederick August II of Saxony.
In 1733 Johann Adolf inherited Saxe-Weissenfels when his brother Christian
died without children.
On November 27th. 1734 Johann Adolf married Fredericka of
Saxe-Gotha-Altenburg. They would have 5 children.
During the Second Silesian War, Prussian troops crossed the Saxon border.
Saxony and Austria made an agreement to fight against Prussia.
In June 1745 they were defeated at the Battle of Hohenfriedberg.
In September 1745 Johann Adolf planned a new campaign, but he changed
his mind. He was replaced as commander-in-chief by
Count Frederick August Rutowski, an illegitimate half- brother of the
Johann Adolf II, Duke of Saxe-Weissenfels died on May 16th. 1746
without surviving male issue. His lands passed to the Electorate of Saxony.