She married a rich Russian tycoon, Anatole Demidov, on November 1, 1840 in Rome. Anatole was raised to the station of Prince by Grand Duke Leopold II of Tuscany shortly before the wedding to fulfill the wishes of Mathilde's father and to preserve Mathilde's station as Princess. Anatole's princely title was never recognised in Russia. They had no children.
The marriage between these two strong and prominent personalities was stormy. Prince Demidoff insisted on keeping his lover, Valentine de St Aldegonde, which of course was fiercely resisted by Mathilde.
Separated since 1843, in September 1846 Mathilde fled to Paris to take refuge with Nieuwerkerke, taking with her the jewels from her dowry. Even so, Demidov was condemned by a tribunal in Saint Petersburg to send Mathilde an annual pension of 200,000 francs and was never able to recover his jewels. In many ways Demidov felt he deserved such punishment, and their separation was authorised in 1847 by a personal decision of tsar Nicholas I.
The terms of the separation announced by the Tribunal in Petersburg forced Anatole to pay annual alimony of 200,000 French francs. Anatole vigorously pursued the return of his property, which led Mathilde and her strong circle of literary friends to mount highly personal and unfair counter-attacks using the public media. In the end, Anatole's heirs never recovered his property since Mathilde's last will was altered towards the end of her life.