Lady Mary Josephine Talbot (nee Crawley) is a character on the series, Downton Abbey. She was played for the entire series run by actress Michelle Dockery.
The eldest....and most hard-hearted daughter
Mary was the oldest of the three daughters of Robert Crawley and Cora Crawley, the Earl and Countess of Grantham. As she was the first born, she was worried about being left out of the inheritance period.
Since her mother's dowry money was tied into an entail, the work of both her late grandfathers, the law was that only male heirs could inherit, and since there were no male heirs directly, it would mean Mary would be shut out.
She was engaged to be married to Patrick Crawley, the son of her second cousin, James. She did not love him (her younger sister, Lady Edith was more in love with Patrick), but Mary was engaged to him, to spite her.
Unfortunately, Patrick and his father were both on board the Titanic and were among those died when it sank. The new heir presumptive was middle-class solicitor, Matthew Crawley, who had just moved to Downton along with his mother, Isobel.
She had mixed relations with her two siblings. Edith and Mary had a lot of animosity, stemming from their nearness in age, and they never liked one another.
However, Mary clearly adored her youngest sister, Lady Sybil Branson. She always called Sybil, "Darling". She would continue using that endearment with her niece, who was named after her, Sybbie Branson. Mary, along with Sybbie's uncle, Kieran Branson, the brother of her father, Tom Branson, were the young girl's godparents.
At first, Mary slowly fell in love with Matthew, but they would fall out after he did not get a straight answer to his engagement to Mary. Mary would do everything possible to undermine Edith.
Mary was harshly affected in the third season when she would lose Sybil to Eclampsia, and then she would lose Matthew, whom she finally married, to a car accident. She had just given birth to her and Matthew's son, George Crawley, whom she would call, Georgie. A will Matthew made, would leave her as half-owner of Downton, finally putting to rest where she would fall in the lines of being the heiress.
She would later try to choose between two suitors. When both of them failed to pan out, she would meet a man named Henry Talbot. Henry was a racing car driver, who was an old friend of Atticus Aldridge, who would later marry Mary's cousin, Lady Rose MacClare. She would drop him, because he was beneath her.
Lady Mary derails Lady Edith!
Mary, out of sheer spite and anger at her family who allowed Henry to stay over, when she did not want that to happen; would derail Edith's engagement to Herbert "Bertie" Pelham, by revealing who Marigold really was.
This was done out of sheer jealousy because Mary did not want to be under her sister, whom she had always considered a failure. Mary wanted to be top of the pecking order, and resented it that Edith was involved with a Marquis.
After her cruelty, Tom and Lady Rosamund Painswick, her aunt, would each severely confront her for what she did. Tom viciously tore his sister in-law to shreds about what she did. Tom finished her off by calling her a coward.
Not long after that, Mary and Edith would later get into a vicious row about what she did. Edith called Mary a "a nasty jealous scheming bitch". Not long after, Edith left for London for a time to throw herself into her work at the magazine. She would have a sympathetic ear in her friend, Laura Edmunds.
After Violet finally got the real reason why she did it, she insisted that Mary get Edith and Bertie back together, which she did, and then she made peace with herself, by reuniting with Henry. The two would eventually marry (with even Edith attending the ceremony), and George would take to Henry beautifully, as he took to George. To George, since he did not remember his birth father, Henry Talbot was his daddy.