The events of the story are put in motion by the marriages of three sisters. Lady Bertram married extremely well to the wealthy baronet Sir Thomas Bertram, while Mrs. Norris married a clergyman, who was given the living at the local parsonage by Sir Thomas; this allows the Norrises to live comfortably, yet far below the opulence of the Bertrams. The third sister, Mrs. Price, married a naval lieutenant who was shortly afterwards wounded in battle and left with a meager pension, scarcely enough to support their eventual household of nine children. Mrs. Norris, always wishing to appear virtuous, proposes that Lady Bertram take one of the children to live with her at Mansfield Park. They choose the eldest daughter Fanny Price, who is the protagonist of the novel. Thus, at age 10, Fanny is sent to live with her wealthy relatives at Mansfield Park.
Fanny's new life is not as she might wish. Her energetic Aunt Norris, who strongly advocated the plan of bringing Fanny when it was first proposed, becomes less interested as time goes on and does little to assist with Fanny's care, except to frequently point out the bother and expense Fanny causes. Aunt Norris refuses to allow a fire to be set in Fanny's room, though Fanny is in poor health.
At Mansfield Park, Fanny grows up with her four older cousins, Tom (17), Edmund (16), Maria (13), and Julia (12), but is always treated as an unwanted poor relation. Only Edmund shows real kindness. He is also the most good-natured of the siblings: Maria and Julia are vain and spoiled, while Tom is an irresponsible gambler. Over time, Fanny's gratitude for Edmund's thoughtfulness secretly grows into romantic love. Lady Bertram is of a lazy and indolent temperament and rarely does anything to assist the raising or monitoring of the children, while all of the children are in awe of Sir Thomas. Mrs. Norris showers attention and affection on her Bertram nieces, particularly Maria, enjoying the prestige and wealth associated with them, but is verbally abusive and mean-spirited toward Fanny, whom she sees as beneath the others. She tries to exclude Fanny from outings and other pleasures.
A few years after Fanny arrives, Aunt Norris is widowed, moves into a cottage of her own, and becomes a constant presence at Mansfield Park. Meanwhile, Tom Bertram, the spendthrift son, incurs a large debt through irresponsible spending and gambling. To pay these debts, Sir Thomas is forced to sell the living of the parsonage, recently freed up by the death of Uncle Norris, to the well-to-do clergyman Dr. Grant, even though Sir Thomas had hoped to keep this lifetime position for his younger son Edmund.
When Fanny is 16, stern patriarch Sir Thomas leaves for a year to deal with problems on his plantation in Antigua. He takes Tom along, hoping the experience will sober him. Meanwhile Mrs. Norris has taken on the task of finding a husband for Maria and manages to introduce her favourite niece to Mr. Rushworth, a very rich man of ₤12,000 a year, but rather weak-willed and stupid. Maria accepts his marriage proposal, subject to Sir Thomas's approval on his return.
After a year in Antigua, Sir Thomas sends Tom home while he continues business alone. Although his wife is indolent almost to the point of disengagement, Sir Thomas feels confident about his family situation, relying on the officious Mrs. Norris and steady, responsible Edmund to keep life running smoothly.
When Fanny is 17, the fashionable, wealthy, and worldly Henry Crawford and his sister, Mary Crawford, arrive at the parsonage to stay with Mrs. Grant, their half-sister. The Crawfords had previously lived with their uncle, Admiral Crawford, and his wife. When his wife died, however, the Admiral brought into the house a mistress, necessitating, for propriety's sake, that Mary Crawford live elsewhere. Unable to persuade her brother to allow her to live with him at his own country estate, she finds a place at Mansfield parsonage with her half-sister Mrs. Grant. The Crawfords are quite a bit wealthier than the Grants, so Mary fears her new situation will be a step down; but she ends up satisfied.
The arrival of the lively, attractive Crawfords disrupts the staid world of Mansfield and sparks a series of romantic entanglements. Mary and Edmund begin to form an attachment, despite her original romantic interest in Tom, who is the heir of Mansfield Park. She is disappointed, though, when she learns that Edmund wants to be a clergyman, which she sees as dull and unambitious. She likes him enough, though, that she is willing to continue their relationship, although Edmund worries that her often cynical conversation may betray a deeper lack of proper principles.
Fanny fears that Mary's charms and attractions have blinded Edmund to her flaws. (Also, of course, Fanny is in love with him herself.) Meanwhile, during a visit to Mr. Rushworth's ancestral estate in Sotherton, Henry deliberately plays with the affections of both Maria and Julia, driving them apart. Maria believes Henry is falling in love with her and treats Mr. Rushworth dismissively, provoking his jealousy. Although nobody is paying much attention to Fanny, she is highly observant and witnesses Maria and Henry flirting indelicately.
Encouraged by Tom and his friend Mr. Yates, the young people decide to put on an amateur performance of Elizabeth Inchbald's play Lovers' Vows. However, Edmund and Fanny both object, believing Sir Thomas would disapprove and feeling that the subject matter of the play, which includes adultery and illegitimacy, is morally compromising. Eventually Edmund reluctantly agrees to take on the role of Anhalt, the lover of the character played by Mary Crawford. Besides giving Mary and Edmund plenty of scope for talking about love and marriage, the play provides a pretext for Henry and Maria to flirt in public. Fanny, however, continues to disapprove of her cousins' activities.
When Sir Thomas unexpectedly arrives home in the middle of a rehearsal, the theatricals are abruptly terminated. Henry, from whom Maria had imminently expected a marriage proposal, instead takes his leave, and she feels crushed, realising that he does not love her. Although she neither likes nor respects Mr. Rushworth, she decides to go ahead and marry him, to escape the oppressive atmosphere of Mansfield and to console her loss of Henry, and they honeymoon in Brighton, taking Julia with them. Meanwhile, Fanny's improved appearance and gentle disposition endear her to Sir Thomas, who begins treating her a bit less distantly. With Maria and Julia gone, Fanny and Mary Crawford are naturally thrown into each other's company. Out of affection and because she knows it will please Edmund and his father, Mary goes out of her way to befriend Fanny.
Henry returns to Mansfield Park and announces to Mary that he intends to amuse himself by making the normally calm, unemotional Fanny fall in love with him. However, Fanny is steadfastly if secretly in love with Edmund, and her indifference piques Henry to fall in love with Fanny instead. To further his suit, he uses his family connections to help Fanny's brother William obtain a commission as a naval lieutenant, to her great joy and gratitude. However, when Henry proposes marriage, Fanny rejects him out of hand, partly because she disapproves of his moral character, and also because she loves someone else. Sir Thomas is astonished and indignant at her refusal, since it is an extremely advantageous match for a poor girl like her. He reproaches her for ingratitude, and believing it is mere timidity or willfulness on Fanny's part, encourages Henry to persevere.
To bring Fanny to her senses, Sir Thomas sends her for a visit to her parents in Portsmouth, hoping that their impecunious circumstances will awaken her to the value of Henry's offer. She sets off with William, who is briefly on leave, and sees him off in his first command. At Portsmouth, she develops a firm bond with her younger sister Susan, but is taken aback by the contrast between her dissolute surroundings — noise, chaos, unpalatable food, crude conversation, and filth everywhere — and the harmonious environment at Mansfield. Henry visits to persuade her that he has changed his frivolous ways and is worthy of her affection. Although Fanny still refuses him, her attitude begins to soften, particularly as Edmund and Mary seem to be moving toward an engagement, which would mean Edmund would be lost to her forever.
Henry leaves for London, and shortly afterward, Fanny learns that scandal has enveloped him and Maria. The two met at a party and rekindled their flirtation, which led to an affair. An indiscreet servant made the affair public, and the story has ended up in the newspapers. The exposure results in Maria running away with Henry. Mr. Rushworth sues Maria for divorce, and the proud Bertram family is devastated. Even worse, Tom has fallen gravely ill as a result of his dissolute lifestyle, and Julia, fearing her father's anger for concealing Maria's affair, has eloped with Tom's flighty friend Mr. Yates.
In the midst of this crisis, Fanny returns to Mansfield Park with her sister Susan, and she is now joyfully welcomed by all the family. A repentant Sir Thomas realises that Fanny was right to reject Henry's proposal and now regards her as his daughter. During an emotional meeting with Mary Crawford, Edmund discovers that Mary does not condemn Henry and Maria's adultery, and only regrets that it was discovered. Her main concern is to cover it up, and she implies that if Fanny had only accepted Henry, there would have been no affair. She also expresses some hope that Tom will not recover from his illness, leaving Edmund to inherit Mansfield Park. Edmund, who had idealised Mary, is devastated to discover at last the sordidness of her true principles. He breaks off the relationship, returns to Mansfield, and goes ahead with plans to be ordained a minister.
While he despairs of ever getting over Mary, Edmund comes to realise how important Fanny is to him. He declares his love for her, and they are married and eventually move to Mansfield parsonage, in the circle of those they love best. Tom recovers from his illness, a steadier and better man for it, and Julia's husband, Mr. Yates, turns out to be a respectable husband after all. Henry Crawford refuses to marry Maria, who is banished by her family to live "in another country", where she is joined by her Aunt Norris. Fanny becomes the effective moral centre of Mansfield Park.