First marriage
Frederica's father was anxious to arrange advantageous marriages for all his daughters, and used family connections to bring this about. Queen Frederika Louisa of Hesse-Darmstadt, wife of King Frederick William II, was a first cousin of Frederica's mother. Frederica's father broached with the Prussian royal family the idea of marriage between their children, and the Prussians were not averse. On 14 March 1793, the Mecklenburg-Strelitz duchesses "coincidentally" met King Frederick William II of Prussia at the Prussian Theatre in Frankfurt-am-Main. He was immediately captivated by the grace and charm of both sisters, Frederica and Louise. The pending marriage negotiations received traction, and within weeks, the matter was settled: Frederica's elder sister Louise would marry Crown Prince Frederick William, and Frederica would marry his younger brother Prince Louis.
The double engagement was celebrated in Darmstadt on 24 April 1793, only a few weeks after the sisters fortuitously met their future father-in-law at the theatre. On 24 December, Duchess Louise and Crown Prince Frederick William were married in the Royal Palace of Berlin; two days later, on 26 December, Duchess Frederica and Prince Louis were also married at the same venue.
Unlike her sister, Frederica did not enjoy a happy marriage. Although her husband died only three years after the wedding, Frederica did bear him three children in as many years: Frederick in 1794; a short-lived son, Charles, in 1795; and a daughter, Frederica, in 1796.
In 1795, King Frederick William II appointed Louis as Chief of the Dragoons Regiment No.1, which was stationed in Schwedt. One year later, on 23 December 1796, Prince Louis died of diphtheria. It was three years almost to the day since their wedding. At this time, his youngest child, Frederica, was less than three months old, and his eldest son was hardly two years old. After Louis's death, his father provided Frederica with a suitable residence near Berlin, and a sufficient income, and she moved with her three children to Schönhausen Palace near Berlin.
In 1797, Frederica became unofficially engaged to her cousin Prince Adolphus, Duke of Cambridge, seventh son of King George III of Great Britain and Queen Charlotte (Frederica's paternal aunt). The Duke of Cambridge asked the consent of his father to the marriage. The King did not refuse his consent but asked his son to wait until the ongoing war with France was over. The relationship eventually ended, with rumors circulating that either Adolphus had offered to release Frederica from the engagement, or – as Queen Charlotte believed – Frederica had jilted him for another man.
