Margaret Theresa was the favourite child of her father, Philip IV of Spain, who referred to her as "my joy" in his private letters.
For political reasons, Margaret Theresa was betrothed as a child to her maternal uncle and paternal cousin, Leopold I, Holy Roman Emperor. Her father stipulated that she should maintain her position in the line of succession to the Spanish throne and would pass her succession rights to her descendants, something Leopold I gladly accepted.
In the summer of 1666, saddened by her father's death, the 15 year old infanta left her native Spain and traveled to Austria. She was accompanied by several Spanish attendants, and was solemnly welcomed by her future husband, her uncle Leopold. Their wedding took place in the city of Vienna, on 12 December 1666. Despite the difference in their ages, some 11 years, they were very happy together, as they shared a number of interests, especially theatre and music. She called him "Uncle" (even after they were married); he called her "Gretl", the German diminutive of the name Margarita.
After giving birth to four children, and weakened by many miscarriages, Margaret Theresa died at the age of 21. Her only surviving child was Archduchess Maria Antonia of Austria, future Electress of Bavaria. Margaret Theresa was buried in the Imperial Crypt, in Vienna.
