Maria von Modena wollte nicht heiraten. Sie wollte Nonne werden. Der Papst, Klemens X., musste sie ermahnen, den Herzog von York zu ehelichen: "... We therefore earnestly exhort you ... to reflect upon the great advantage which would accrue to the Catholic faith in the aforementioned Kingdom [England und Schottland] through your marriage, and trust that, inflamed with zeal for the good which may result, you will open to yourself a field of merit wider than that of the virginal cloister." (in: Maureen Waller: Ungrateful Daughters – The Stuart Princesses who stole their father’s crown, id., p. 16).
Maria fügte sich dem päpstlichen Befehl und heiratete am 19. oder 30. September 1673 per procurationem in ihrem väterlichen Schloss den Herzog von York. Als Stellvertreter für den abwesenden Bräutigam diente Henry Mordaunt, der Graf von Peterborough. "... when the time for her departure for England was named, she cried bitterly for two whole days and nights, and would not be pacified till her mother consented to accompany her … Maria left Modena about a fortnight after the solemnization of her marriage, accompanied by her mother, her brother, the young Duke of Modena, her uncle, Prince Rinaldo D’Esté, and a princely train of nobles. … but on reaching the border of France, her brother and most of the nobles who had attended her out of respect to the house of Esté, returned. At Paris, Louis the Fourteenth and the most exalted and illustrious personages in France entertained her with regal magnificence, and treated her with all conceivable honour and distinction. These princely favours she returned with becoming grace and dignity, and was about to set out for England, when a violent attack of fever laid her up and forced her to keep her bed for a fortnight." (in: Francis Lancelott: The Queens of England and their Times, Volume II, id., p. 745).
Mit ihrer Mutter Laura Martinozzi war Maria also an ihrem 15. Geburtstag, am 5. Oktober 1673, nach England aufgebrochen. Einen Zwischenstopp hatte man in Frankreich eingelegt: "In France Louis XIV, who had done much to promote the marriage, gave her a splendid reception and took her on a tour of his new palace at Versailles. One of the reasons that the marriage was so unpopular with the English Parliament was that it was deemed to be part of the hated French alliance, and, indeed, Louis XIV was providing a proportion of the dowry. While Parliament was raging against the marriage in England, the bride was delayed in France owing to a bout of dysentery. Eventually, on 21 November, she sailed to Dover on the yacht Catherine. James, with a small party of attendants, stood on the shore [in Dover] to meet her. As the young Duchess, tired and shaken by the rough crossing, stepped out of the boat, James came forward to greet her. She promptly burst into tears. To the fifteen-year-old girl, her forty-year-old bridegroom must have looked impossibly old. James was tall and fair like his grandmother Anne of Denmark, but his once handsome face betrayed years of debauchery and there was a sneer, a cruel curl to his lip." (in: Maureen Waller: Ungrateful Daughters – The Stuart Princesses who stole their father’s crown, id., p. 18). " ... at the same night the marriage was lawfully consummated." (in: Francis Lancelott: The Queens of England and their Times, Volume II, id., p. 746).
Mary Beatrice [Maria von Modena] became particularly fond of the Lady Mary [ihre ältere Stieftochter], who was closer to her age. Mary was charming, affectionate and intelligent, and the two became friends. Tall and graceful like herself, Mary loved to dance. Mary Beatrice might have found it harder to warm to the younger girl, who was less open and more suspicious than her sister. Lady Anne [ihre jüngere Stieftochter] had a slight cast in her eye, so that she appeared to be looking at a person askance, which was rather disconcerting. Subjected to her mother's strict discipline, Mary Beatrice had received a far superior education to that given these Stuart Princesses. Besides Italian, she was fluent in French and Latin and soon spoke and wrote the English language better than they did." (in: Maureen Waller: Ungrateful Daughters – The Stuart Princesses who stole their father’s crown, id., pp. 22-23).
Nachdem ihre Mutter am 30. Dezember 1673 England verlassen und sich nach Modena zurückbegeben hatte, entdeckte Maria die Liebe zu ihrem "alten" Gatten: "Once Mary Beatrice was left alone with her husband, something curious happened. She started to fall in love with him. 'After her [mother's] departure,' she recalled many years later, 'I became very fond of my husband, and my affection for him increased with every year that we lived together. It was sinful for any one to love an earthly creature as I loved him.' She recognised that he was 'a very good man, and wishes me well and would do anything to prove it to me.' She drew consolation from the fact that he was a good Catholic, 'so firm and steady in our holy religion'. She was not to be consoled for long. Mary Beatrice had been in England for four months when she was incensed to discover that she was not the only object of the Duke's affections. She learned that James had a mistress, Arabella Churchill, and that she had just given birth to their fourth child." (in: Maureen Waller: Ungrateful Daughters – The Stuart Princesses who stole their father’s crown, id., pp. 24-25).
"On the tenth of January, 1675, exactly twenty-five minutes after four in the afternoon, Maria gave birth to her first child, a daughter, at St. James’s Palace. Katherine Laura … was a delicate child, and, to the great grief of her parents, died of a convulsion fit, in the tenth month of her age, and was buried on the fifth of October, 1675, in the vault of Mary, Queen of Scots, in Westminster Abbey. ... On the eighteenth of August, 1676, at five minutes past eight in the morning, Maria’s second child – a daughter – entered the world. The infant was christened Isabella … the Duchess was safely delivered of a prince [am 7. November 1677], to the great joy of the whole court … The child is but little, but sprightly, and likely to live. The evening after his birth, he was christened with great pomp … by the name of Charles, after the King, who stood godfather, and created him Duke of Cambridge. … The infant Charles, although a healthy babe, was short-lived; four days after his birth the smallpox broke out at St. James’s, he caught the infection … and on the eleventh of December he died in a convulsion fit." (in: Francis Lancelott: The Queens of England and their Times, Volume II, id., p. 748).
Maria erlitt mindestens eine Fehlgeburt im Jahr 1674 und fünf Totgeburten. Sie brachte ihrem Gatten folgende Kinder zur Welt:
- die Tochter Katharina Laura, geboren am 10. Januar 1675 und gestorben am 3. Oktober 1675
- die Tochter Isabella, geboren am 18. oder 28. August 1676, gestorben am 2. oder 4. März 1681
- den Sohn Karl, Herzog von Cambridge, geboren am 7. November 1677, gestorben am 11. oder 12. Dezember 1677 an den Pocken
- die Tochter Elizabeth, geboren und gestorben um 1678
- die Tochter Charlotte Maria, geboren am 15. oder 16. August 1682, gestorben am 16. Oktober 1682
- den Sohn Jakob, Prinz von Wales (The Old Pretender), geboren am 10. Juni 1688, gestorben am 1. Januar 1766
- die Tochter Louisa Maria Theresia, geboren am 28. Juni 1692 und gestorben am 18. April 1712
Maria verlor viele ihrer Kinder, als sie noch sehr jung waren. Sie scheint zudem viele Frühgeburten gehabt zu haben. Daher hatten ihre Kinder kaum eine Chance alt zu werden. An eine Nonne in ihrem Kloster in Modena schrieb sie gegen Ende des Jahres 1682: "But I console myself with the thought that I have more angels to pray for me, and I ought to esteem myself honoured that, while other women give their children to the world, I have given all mine to God, in whose mercy I still hope that He may some day comfort me by giving me a male child who shall live, and yet in the end gain Heaven." (in: Maureen Waller: Ungrateful Daughters – The Stuart Princesses who stole their father’s crown, id., p. 36).