His parents were François de Rohan and Anne Julie Rohan-Chabot; his mother had for a time been the mistress of Louis XIV which led to speculation as to his younger brother's real paternity. It was his mother who bought the Lordship of Soubise to the family. Hercule Mériadec was the second son and as such was given the lot of being in his older brother's shadow. However, that all changed when his brother died unexpectedly at the age of 22 leaving Hercule Mériadec as the heir of the family.
In 1710 Hercule Mériadec became Duc de Rohan-Rohan; that same year also saw the birth of his first grand-son, Charles, who would one day succeed him. Five years later his family was given the further distinction of having his wife appointed governess to the child-king Louis XV.
In 1724 the only son and heir to Hercule Mériadec died alongside his wife - both of smallpox. This meant that the then fourteen year old Charles was left orphaned but not abandoned. Hercule Mériadec took the young boy into his own personal care and raised him at court. There Charles became the close friend of Louis XV probably to joy of his grand-father. Sadly, Anne Genviève died just three years after her son leaving Hercule Mériadec a widower.
Following the death of his wife Hercule Mériadec decided to have his town-house, the hôtel de Soubise, redone in the modern style which has survived to this day.
Hercule Mériadec was in no rush to remarry. Actually, he remained unmarried for 27 years before he wed Marie Sophie de Courcillon on 2 September 1732. Unlike his first marriage this match produced no children but at this point the line of succession was already secured.
Hercule Mériadec died at the ripe old age of 79 in 1749.