Some thirty years before the time of the narrative, and eleven years before the start of the events which are to be recounted, a young woman named Maria Ward married the wealthy and titled Sir Thomas Bertram of Mansfield Park. The new Lady Bertram became the talk of the neighborhood for her marriage; although her family was comfortable, they were not wealthy enough to see one of their daughters married to a baronet under normal circumstances. Hopes were high for her sisters, but one of them ended up marrying a clergyman, the Rev. Norris, and the other one a sailor, who was soon injured in the line of duty and came home to drink and father children. Mrs. Norris, the reverend's wife, lives near her sister Lady Bertram; Rev. Norris is the minister to the parish attached to Mansfield Park. The two women have not heard from their third sister for many years, until, one day, a letter arrives informing them that she is about to give birth yet again and begging them to help see her older children placed in the world. Motivated by a sense of self-importance rather than any real family feeling, Mrs. Norris, Lady Bertram, and Sir Thomas decide to send for the wayward woman's oldest daughter, a girl of nine named Fanny. After a discussion of the girl's proper "place" in the Bertram household, during which Mrs. Norris, ever the busybody, points out that she must be constantly reminded of her lower status, they decide that she will live with the Bertrams rather than the childless Mrs. Norris, who claims that she has no money and that her husband will be bothered by the presence of a child.
Fanny arrives at Mansfield Park and meets the family. Sir Thomas and Lady Bertram have four children: Tom, the heir, is 17; Edmund, who is to be a clergyman, is 16; Maria is 13; and Julia is 12. Fanny is quite shy and is frightened by brash Sir Thomas, neurotic Lady Bertram, and the spoiled children. The family is pleased with Fanny's modest looks and her retiring personality; she already seems to "know her place." Maria and Julia are not impressed with their new playmate, as she has no fancy dresses and does not speak French, nor is she interested in hearing their musical efforts. Nevertheless, they are happy to have her around to use as a political third in their childish skirmishes. Mrs. Norris's constant harangues and the cruelties of the governess and the two girls soon wear Fanny down. One day, Edmund finds Fanny crying in the stairway. He comforts her, and the two become fast friends. Soon Fanny becomes almost happy at Mansfield Park, due in large part to the companionship of her cousin Edmund. Maria and Julia continue to see her as a second-class citizen, though; they are particularly puzzled that she does not want to learn drawing or music. Altogether, though, everyone is quite pleased with the young lady that Fanny is becoming. Sir Thomas also goes out of his way to help Fanny's beloved brother William, for whom he procures a spot in the Navy.
After Fanny has been at Mansfield Park for about five years, Mrs. Norris's husband dies. His position as parish minister was to have gone to Edmund, but Edmund is not of age yet and has not taken his orders as a priest. Normally, the position would go to a family friend to hold until Edmund was old enough, but Tom Bertram has been extravagant and contracted many debts, necessitating that the living (as parish positions are called in the 19th century) go to someone else. Dr. Grant comes with his wife to Mansfield to fill the position. Everyone expects that Mrs. Norris, now a widow, will take Fanny to live with her, but by her usual convoluted logic, she convinces the Bertrams, much to Fanny's relief, to keep the girl with them. The Grants soon settle into Mansfield society, despite Mrs. Norris's criticisms of their habits of household management.
About a year after the Grants' arrival, Sir Thomas finds it necessary to go to Antigua (an island in the Caribbean) to take care of some business matters concerning his plantation holdings there. He departs, taking Tom with him. Most of the inhabitants of Mansfield Park are secretly happy to see Sir Thomas go; his daughters view him as a stern master who thwarts their girlish pleasures, and Fanny is mostly afraid of him. When he leaves, she "grieve[s] because she could not grieve" sincerely at his departure; although he has told her to invite her brother William to visit Mansfield Park when his ship returns to England, he has also remarked that she herself has changed little over the last six years.
The introductory chapters of this novel signify that Mansfield Park will take as its subject social mobility, which also happens to be the subject of all of Jane Austen's other novels and, for that matter, the subject of most of the novels written in the eighteenth and nineteenth centuries. There is a twist here, however. The marriage of Sir Thomas and Lady Bertram is the kind of event that would normally conclude one of Austen's novels: a beautiful but not economically suitable girl captures the heart and eventually the hand of a nobleman, while her younger sister "settles" for a nice clergyman with a comfortable living. Here we see for the first time the aftermath. Lady Bertram is a neurotic hypochondriac who sleeps most of the day, while Mrs. Norris is a busybody and one of the most excruciatingly annoying characters in all of fiction, truly a testament to Austen's wit and creative power. When Fanny comes to Mansfield Park, we see the true results of the social mobility of the previous generation. Although they want to help their sister's children out of a sense of self-importance and misguided noblesse oblige, Mrs. Norris and Lady Bertram do not want to see the rest of the family make the same jump in status they themselves have, particularly since Fanny, by virtue of her father's position in life, would be making a much greater leap. Those who have enjoyed the possibilities of social mobility have now become the most conservative, the zealous guardians of the class system. This is most evident in Mrs. Norris, who hasn't made it to the top herself but is the most interested in preserving the honor of Sir Thomas's family name, to continue to enjoy the glory of her own connection with Mansfield Park.
Fanny is a small child when she comes to Mansfield, though. Her initial moves along the social ladder will involve not marriage but the adoption of surrogate parents. Here she has something in common with the orphans of Dickens's novels, although, since she still has living parents, she cannot be the kind of social blank slate that Dickens' characters are. The insistence of Mrs. Norris and Sir Thomas that she must become enough of a sister that Edmund will not think of marrying her, but not enough to think herself the equal of the Bertram girls, points out the paradox. Fanny's attitude toward her adopted siblings by the end of the novel will speak to the relative success of her "adoption" as a Bertram.