She became part of the household of Prince Alexander Menshikov, who was the best friend of Peter the Great of Russia. Anecdotal sources suggest that she was purchased by him. Whether the two of them were lovers is disputed, as Menshikov was already engaged to Darya Arsenyeva, his future wife. It is clear that Menshikov and Marta formed a lifetime alliance, and 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.[citation needed] 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).[2] She and Darya Menshikova accompanied Peter and Menshikov on their military excursions.
Though no record exists, Catherine and Peter are described as having married secretly between 23 Oct and 1 Dec 1707 in St. Petersburg. They had twelve children, two of whom survived into adulthood, Yelizaveta (born 1709) and Anna (born 1708). Peter had moved the capital to St Petersburg in 1703. While the city was being built he lived in a three-room log cabin with Catherine, where she did the cooking and caring for the children, and he tended a garden as though they were an ordinary couple. The relationship was the most successful of Peter's life and a great number of letters exist demonstrating the strong affection between Catherine and Peter. As a person she was very energetic, compassionate, charming and always cheerful. She was able to calm Peter in his frequent rages and was called in to attend him during his epileptic seizures.
Catherine continued to accompany Peter on his Pruth Campaign in 1711. There Catherine was said to have saved Peter and his Empire, as related by Voltaire in his book Peter the Great. Surrounded by overwhelming numbers of Turkish troops, Catherine suggested before surrendering, that her jewels and those of the other women be used in an effort to bribe the Ottoman grand vizier Baltacı Mehmet Pasha into allowing a retreat. Mehmet allowed the retreat, whether motivated by the bribe or considerations of trade and diplomacy. In any case Peter credited Catherine and proceeded to marry her again (this time officially) at Saint Isaac's Cathedral in St. Petersburg on 9 February 1712. Catherine was Peter's second wife; he had previously married and divorced Eudoxia Lopukhina, who had borne him the Tsarevich Alexis Petrovich. Upon their wedding, Catherine took the style of her husband and became Tsarina. When Peter elevated the Russian Tsardom to Empire, Catherine became Empress. The Order of Saint Catherine was instituted by her husband on the occasion of their wedding.