Julius Siegmund was the fourth son of the Duke Silvius I Nimrod of Württemberg-Oels (1622–1664) from his marriage to Elisabeth Marie, Duchess of Oels (1625–1686) .
After his father's death in 1664, his mother initially ruled the Duchy of Oels as regent for her four sons.
The sons went on their Grand Tour and visited, among other countries, the Netherlands, where the eldest brother Charles Ferdinand died in 1669.
Julius Siegmund was a member of the Fruitbearing Society, under the nickname der Unverwelkte ("the unwithered one").
Julius Siegmund married on 4 April 1677 in Grabow with Anna Sophia (1647–1726), the daughter of the Adolf Frederick I of Mecklenburg-Schwerin.