Catherine I of Russia wife. It is clear that Menshikov and Marta formed a lifetime alliance. It is possible that Menshikov, who was quite jealous of Peter's attentions and knew his tastes, wanted to procure a mistress on whom he could rely. In any case, in 1703, while visiting Menshikov at his home, Peter met Marta. In 1704, she was well established in the Tsar's household as his mistress, and gave birth to a son, Peter. In 1705, she converted to Orthodoxy and took the new name of Catherine Alexeyevna ("Yekaterina Alexeyevna"). She and Darya Menshikova accompanied Peter and Menshikov on their military excursions.
Tsarevna Catherine Ivanovna of Russia V's daughters, she seems to have been the most capable. In 1708 the family moved to the new capital, Saint Petersburg. According to contemporaries, Catherine was a short, dark-haired, and pale beauty and was a popular socialite with her charm and sociability. At the request of her uncle Peter I, she was married on 19 April 1716 in Danzig to Karl Leopold, Duke of Mecklenburg-Schwerin. He had initially proposed to Catherine's sister Anna (then Dowager Duchess of Courland) but Peter I instead chose Catherine to be his bride. The marriage created a political alliance between Russia and Mecklenburg against Sweden,