Born in the German town of Kassel, Tethart Haag moved together with his father, Johann Haag in 1747 to Holland when his father was employed as court painter to the Stadhouderlijk Hof in Leeuwarden. When Prince William IV was elevated to Stadtholder in 1747 and moved to The Hague, Johann took his son with him to move there. Tethart was taught by his father in painting as a portrait and horse painter. In 1760 Tethart followed his deceased father as court painter to Stadtholder Prince William V. Haag also managed his collection of paintings and was his chief adviser in the foundation in 1774 of the stadtholder 'Picture Gallery' at the Buitenhof in The Hague. He was thus the first director and curator of this first public museum in the Netherlands, which would later be known as the Prince William V Gallery.