He is best known for discovering photosynthesis by showing that light is essential to the process by which green plants absorb carbon dioxide and release oxygen.

[5] In his lifetime he was known for successfully inoculating the members of the Habsburg family in Vienna against smallpox in 1768 and subsequently being the private counsellor and personal physician to the Austrian Empress Maria Theresa.

He studied for two more years at the University of Leiden, where he attended lectures by, among others, Pieter van Musschenbroek, which led Ingenhousz to have a lifelong interest in electricity.

Following his father's death in July 1764, Ingenhousz intended to travel through Europe for study, starting in England where he wanted to learn the latest techniques in inoculation against smallpox.

Via the physician John Pringle, who had been a family friend since the 1740s, he quickly made many valuable contacts in London, and in due time became a master inoculator.

In 1768, Empress Maria Theresa read a letter by Pringle on the success in the fight against smallpox in England, whereas in the Austrian Empire the medical establishment vehemently opposed inoculations.

He had planned to inoculate the Royal Family by pricking them with a needle and thread that were coated with smallpox germs taken from the pus of a smallpox-infected person.