The couple had traveled to Russia to visit Frederick's sister Sophie, who was married to the heir to the Russian throne, the Tsesarevich Paul.

In 1797, Frederick married Charlotte, Princess Royal, eldest daughter of King George III of the United Kingdom, who supervised the education of Paul and his two surviving siblings, Wilhelm and Catharina.

They had five children: Shortly before his marriage, Paul had a mistress, an actress named Friederike Margarethe Porth (22 August 1776, Halberstadt - 9 June 1860, Frankfurt am Main).

On 16 February 1836, in Augsburg, Karoline married Freiherr Karl Pfeffel von Kriegelstein (22 November 1811, Dresden - 25 January 1890, Munich).

Shortly after the death of his wife in 1847, Paul went to England with his long-term mistress Magdalena Fausta Angela Creus y Soler, of Menorcan origin, the widow of Sir Sandford Whittingham KCB, and they were married in the Parish Church of St Nicholas, Brighton, Sussex, on 26 April 1848.