Heir of Hessen-Kassel, he married Princess Luise of Prussia in 1700. However, when she died in 1705 they were childless. Friedrich proved to be a gifted soldier in the War of the Spanish Succession and had an outgoing manner which won him many friends. 
In 1715 he married Ulrica Eleonore, sister of Carl XII, King of Sweden, but this marriage, too, remained childless. He remained a loyal friend to Carl XII but never learned to speak Swedish. In 1718 he ensured his wife's election to the Swedish crown, overriding the stronger claim of her eighteen-year-old nephew. However, in 1720 she abdicated in his favour and he succeeded to a throne to which he had no hereditary right. 
Sweden's political leaders had constitutionally deprived the crown of power and, as a result, Friedrich increasingly lost interest in government, devoting his time to hunting and his mistresses, 
especially Hedvig Taube. 
In 1730 he succeeded as Landgraf of Hessen-Kassel but appointed his brother and heir, Wilhelm, as his regent. After the war with Russia, from 1741 till 1743, Sweden was forced to accept Adolf 
Frederik of Holstein-Gottorp as Friedrich's heir.
 

Source: Leo van de Pas