John George succeeded his father as elector of Saxony when he died, in 1680; he was also appointed Marshal of the Holy Roman Empire.

In 1685 John George III met the Venetian opera singer Margarita Salicola and began a relationship; he brought her to Dresden (not only to work, but also as his official mistress).

John George showed a strong interest in the military and even while he was still the heir led Saxon Army forces in the Rhine Campaign.

[clarification needed] After his accession as Elector, he reduced the size of the royal household and began with the establishment of a small standing army of 12,000 men, after the model of the Margraviate of Brandenburg and managed to extract from the states of the realm a commitment to contribute funds.

There was also the matter of John George's wish for Emperor Leopold I to decide a law case concerning a wooded area in the Ore Mountains (Erzgebirge) in his favour.

But on 15 September, without taking leave of the Emperor or the other commanders, he set off with his troops on the march back to Saxony, probably as a result of the brusque treatment he had been accorded as a Protestant.

He later had to leave the theatre because of an illness but, against the advice of his physicians and advisors, he returned in May 1690 and with a reinforced alliance with the Emperor, took overall command of the imperial army.

Success was limited, however, partly owing to personal skirmishes between John George, the Field Marshal Hans Adam von Schöning and the Austrian commander Aeneas de Caprara; only the crossing of the Rhine at Sandhofen succeeded.

John George married Anne Sophie, daughter of King Frederick III of Denmark and Norway, in Copenhagen on 9 October 1666.