He was Count of Wied-Neuwied from September 17, 1737 to May 29, 1784, when he was elevated to Prince and continued to rule in that capacity until his death on August 8, 1791.

In 1737, after his studies in Strasbourg and Königsberg, he was instrumental in the negotiation of the Vienna peace, which ended the War of the Polish Succession.

He promoted the construction of a Mennonite church and a synagogue in Neuwied and wholly implemented the grandfather Frederick III's tolerant religious policy.

As Chairman of the Lower Rhenish-Westphalian Count College, Johann Friedrich Alexander was able to play a role in national politics.

His success in mediating the dispute between the Protestant and Catholic Imperial Count led to his elevation to the rank of prince.