His first marriage took place in 1762 when he married Louise-Pauline de Gand de Mérode but the union was to remain without children. That same year also brought him the title of Duc de La Rochefoucauld with the death of his grandfather (his own father having died earlier). Eighteen years later Louis Alexandre found himself a widower still without an heir and decided to marry Alexandrine-Charlotte de Rohan-Chabot. What hopes he must have had regarding fathering an heir is unknown but his second marriage also remained without issue.
Through these years of marriage it would seem that Louis Alexandre fostered a keen interest in the natural sciences. Eventually, this led to him presiding as President of both the Royal Academy of Sciences and the Royal Society of Medicine.
Besides his love for the sciences, the Duc was a well-travelled man. In his youth he had made his grand tour of Europe which brought him through Germany, Sweden, Savoy, England, Switzerland and Italy.
After his second marriage, Louis Alexandre turned his attention to helping his close friend, Benjamin Franklin, whom he had translated several literary works for before. As it happens, Louis Alexandre was a great supporter of the American side in their war against Britain. Considering the circle which Louis Alexandre moved in he appeared to have been interested in some of the new enlightenment ideas. For example his circle of friends includes names such as Turgot and Desmarest.
By 1789 the revolution was stirring and Louis Alexandre took his seat as a member of the States General. Having originally supported the third estate - he had been one of 47 aristocrats who had openly laid their weight behind the third estate - the drastic turn of events unnerved the otherwise liberal Duc. He managed to keep a place in politics after the fall of the first Assembly but then made the political mistake of opposing Pétion.
Things were now getting too dangerous for Louis Alexandre to remain in Paris. Despite his well-known sympathies for the revolutionary cause he was still a nobleman and a high-ranking one at that. On 10 August 1792 he abruptly resigned his position and fled France to avoid the increasing hostile atmosphere. Despite his efforts, Louis Alexandre never made it out of France. At Gisors he was apprehended while trying to get his wife and mother to safety.
On 4 September Louis Alexandre was murdered by militants who had assembled to fight the Prussians - their other main task was to hunt down fleeing aristocrats.