As feared by the reformers, Mary did begin to reverse the legislation passed during her half-brother's reign and restore as much of the old order as she believed possible. She was cautious, because she did not want to provoke religious disorder and strove, instead, to find a balance. She reinstated some of the Catholic bishops and imprisoned certain zealous reformers, but she did not dare overturn her father's enactment which would have restored the Pope's supremacy. She did, however, pass an act, which invalidated her father's divorce from Katherine of Aragon, restoring Mary's legitimacy and automatically bastardising Elizabeth.

Mary had never married; in fact she had come to regard herself as a spinster. It is true that she was not overly attractive and she had inherited congenital syphilis from her father which not only gave her a weak constitution, with regular headaches and poor eyesight, but gave her a form of rhinitis which meant that her breath was always foul-smelling. This hardly endeared her to anyone and if there were to be any marriage at all it would be a political one. Her choice settled on Philip of Spain, the son of the Holy Roman Emperor, Charles V. Parliament petitioned the Queen to reconsider and to seek a husband from within England, but Mary steadfastly refused. Her choice of Philip was extremely unpopular, with many believing that Philip would use the opportunity to enforce his own control over England and that Mary would become a Spanish puppet. A group of conspirators including Sir James Crofts and Sir Thomas Wyatt plotted against Mary but the intended coup was ill planned and only Wyatt's rebellion in Kent, which began on 25 January 1554, carried any force. It was, however, soon quashed, many of the ringleaders were captured, tried and executed. The Princess Elizabeth was also implicated in the coup and briefly confined to the Tower of London. Mary's marriage with Philip went ahead in July 1554. In accordance with Spanish tradition, Philip was granted the title of king. He is the only English king consort. Eighteen months later Philip's father abdicated, relinquishing to him the throne of Spain. Mary likewise assumed the title of Queen of Spain. It is also worth an aside here and noting that when Mary Queen of Scots married Francois, the French Dauphin in April 1558, he likewise became king consort of Scotland. Had Mary Tudor lived another eight months then England would have been ruled by a king and queen of Spain at the same time that Scotland was ruled by a king and queen of France. Although Mary fell in love with Philip, the love was not reciprocated. Philip left for Spain in August 1555, returning to England only once between March and June 1557. His treatment of Mary was callous and left her heart-broken.

In November 1554 Cardinal Reginald Pole, the papal legate (and subsequently archbishop of Canterbury) announced that England had been absolved of papal censures and was restored to the Holy See. Whilst this was welcomed by many, in its wake came the papal requirement that all heretics must be burned at the stake. It was from this moment that Mary's reign of terror began, which earned her the title of Bloody Mary. Almost certainly much of it was not of her own desire, but between February 1555 and November 1558 almost three hundred victims perished cruelly in the flames. These included Thomas Cranmer, archbishop of Canterbury, and the famous example of Hugh Latimer, bishop of Worcester and Nicholas Ridley, bishop of London, who were burned outside Balliol College, Oxford. These executions were recorded and published, during Elizabeth's reign, by John Foxe in his best selling Acts and Monuments of these latter and perilous days (1563), now popularly known as Foxe's Book of Martyrs.

Another conspiracy was hatched against Mary in December 1555, called the Dudley Conspiracy after one of its main architects, Sir Henry Dudley. The idea was to rob the finances from the Exchequer, depose Mary and Philip, and raise Elizabeth to the throne. It was also intended to secure her marriage to Edward Courtenay, a distant relative descended from Edward IV. However before the plot could be achieved news of it leaked out and by March 1556 most of the conspirators were either arrested or had fled to France. There were other plots, all of which failed, but England remained in a sorry state. The last blow to national pride came when Philip convinced Mary and the English Parliament to join Spain in its war against France. As a result of the conflict England lost Calais, its last possession in France.

Mary's husband had returned to Spain and never came back to her. Although she believed she might be pregnant, she had never conceived. Both of these facts caused a depression which added to her overall ill-health, and the loss of Calais as well as all of the Protestant persecutions caused Mary to regard her reign as a total failure. Her famous remark, "when I am dead, you will find Philip and Calais engraved upon my heart", was not without substance. She died in the early hours of 17 November 1558, her constitution further weakened by influenza. A few weeks earlier Mary had reluctantly conceded that her half-sister, Elizabeth, would be her successor.