The Prophet (peace and blessings of Allaah be upon him) did not marry Mariyah al-Qibtiyyah, rather she was a concubine who was given to him by al-Muqawqis, the ruler of Egypt. That took place after the treaty of al-Hudaybiyah. Mariyah al-Qibtiyyah was a Christian, then she became Muslim (may Allaah be pleased with her). 

Ibn Sa’d said: 

The Messenger of Allaah (peace and blessings of Allaah be upon him) lodged her – meaning Mariyah al-Qibtiyyah and her sister – with Umm Sulaym bint Milhaan, and the Messenger of Allaah (peace and blessings of Allaah be upon him) entered upon them and told them about Islam. He took Mariyah as a concubine and moved her to some property of his in al-‘Awaali… and she became a good Muslim. 

Al-Tabaqaat al-Kubra, 1/134-135 

Ibn ‘Abd al-Barr said: 

Mariyah died during the caliphate of ‘Umar ibn al-Khattaab, in Muharram of 16 AH. ‘Umar gathered the people himself to attend her funeral, and he led the funeral prayer for her. She was buried in al-Baqee’. 

Al-Isti’aab, 4/ 1912 

Mariyah (may Allaah be pleased with her) was one of the Prophet’s concubines, not one of his wives. The Mothers of the Believers are the wives of the Prophet (peace and blessings of Allaah be upon him). Allaah says (interpretation of the meaning): 

“The Prophet is closer to the believers than their ownselves, and his wives are their (believers’) mothers (as regards respect and marriage)”

[al-Ahzaab 33:6] 

The Prophet (peace and blessings of Allaah be upon him) had four concubines, one of whom was Mariyah. 

Ibn al-Qayyim said: 

Abu ‘Ubaydah said: He had four (concubines): Mariyah, who was the mother of his son Ibraaheem; Rayhaanah; another beautiful slave woman whom he acquired as a prisoner of war; and a slave woman who was given to him by Zaynab bint Jahsh. 

Zaad al-Ma’aad, 1/114 

For more information of the wives of the Prophet (peace and blessings of Allaah be upon him) – the Mothers of the Believers – please see the answer to question no. 47072 

And Allaah knows best.