- Willie Stark's political right-hand man, the narrator of the novel and in many ways its protagonist. Jack comes from a prominent family (the town he grew up in, Burden's Landing, was named for his ancestors), and knows many of the most important people in the state. Despite his aristocratic background, Jack allies himself with the liberal, amoral Governor Stark, to the displeasure of his family and friends. He uses his considerable skills as a researcher to uncover the secrets of Willie's political enemies. Jack was once married to Lois Seager, but has left her by the time of the novel. Jack's main characteristics are his intelligence and his curious lack of ambition; he seems to have no agency of his own, and for the most part he is content to take his direction from Willie. Jack is also continually troubled by the question of motive and responsibility in history: he quit working on his PhD thesis in history when he decided he could not comprehend Cass Mastern's motives. He develops the Great Twitch theory to convince himself that no one can be held responsible for anything that happens. During the course of the novel, however, Jack rejects the Great Twitch theory and accepts the idea of responsibility.
- Jack Burden's boss, who rises from poverty to become the governor of his state and its most powerful political figure. Willie takes control of the state through a combination of political reform (he institutes sweeping liberal measures designed to tax the rich and ease the burden on the state's many poor farmers) and underhanded guile (he blackmails and bullies his enemies into submission). While Jack is intelligent and inactive, Willie is essentially all motive power and direction. The extent of his moral philosophy is his belief that everyone and everything is bad, and that moral action involves making goodness out of the badness. Willie is married to Lucy Stark, with whom he has a son, Tom. But his voracious sexual appetite leads him into a number of affairs, including one with Sadie Burke and one with Anne Stanton. Willie is murdered by Adam Stanton toward the end of the novel.
- Jack Burden's first love, Adam Stanton's sister, and, for a time, Willie Stark's mistress. The daughter of Governor Stanton, Anne is raised to believe in a strict moral code, a belief which is threatened and nearly shattered when Jack shows her proof of her father's wrongdoing.
- A brilliant surgeon and Jack Burden's closest childhood friend. Anne Stanton's brother. Jack persuades Adam to put aside his moral reservations about Willie and become director of the new hospital Willie is building, and Adam later cares for Tom Stark after his injury. But two revelations combine to shatter Adam's worldview: he learns that his father illegally protected Judge Irwin after he took a bribe, and he learns that his sister has become Willie Stark's lover. Driven mad with the knowledge, Adam assassinates Willie in the lobby of the Capitol towards the end of the novel.
Judge Montague Irwin
- A prominent citizen of Burden's Landing and a former state Attorney General; also a friend to the Scholarly Attorney and a father figure to Jack. When Judge Irwin supports one of Willie's political enemies in a Senate election, Willie orders Jack to dig up some information on the judge. Jack discovers that his old friend accepted a bribe from the American Electric Power Company in 1913 to save his plantation. (In return for the money, the judge dismissed a case against the Southern Belle Fuel Company, a sister corporation to American Electric.) When he confronts the judge with this information, the judge commits suicide; when Jack learns of the suicide from his mother, he also learns that Judge Irwin was his real father.
- Willie Stark's secretary, and also his mistress. Sadie has been with Willie from the beginning, and believes that she made him what he is. Despite the fact that he is a married man, she becomes extremely jealous of his relationships with other women, and they often have long, passionate fights. Sadie is tough, cynical, and extremely vulnerable; when Willie announces that he is leaving her to go back to Lucy, she tells Tiny Duffy in a fit of rage that Willie is sleeping with Anne Stanton. Tiny tells Adam Stanton, who assassinates Willie. Believing herself to be responsible for Willie's death, Sadie checks into a sanitarium.
- Lieutenant-Governor of the state when Willie is assassinated. Fat, obsequious, and untrustworthy, Tiny swallows Willie's abuse and contempt for years, but finally tells Adam Stanton that Willie is sleeping with Anne. When Adam murders Willie, Tiny becomes Governor.
- Willie Stark's driver, and also his bodyguard--Sugar-Boy is a crack shot with a .38 special and a brilliant driver. A stuttering Irishman, Sugar-Boy follows Willie blindly.
- Willie's long-suffering wife, who is constantly disappointed by her husband's failure to live up to her moral standards. Lucy eventually leaves Willie to live at her sister's poultry farm. They are in the process of reconciling when Willie is murdered.
- Willie's arrogant, hedonistic son, a football star for the state university. Tom lives a life of drunkenness and promiscuity before he breaks his neck in a football accident. Permanently paralyzed, he dies of pneumonia shortly thereafter. Tom is accused of impregnating Sibyl Frey, whose child is adopted by Lucy at the end of the novel.
- A beautiful, "famished-cheeked" woman from Arkansas, Jack's mother is brought back to Burden's Landing by the Scholarly Attorney, but falls in love with Judge Irwin and begins an affair with him; Jack is a product of that affair. After the Scholarly Attorney leaves her, she marries a succession of men (the Tycoon, the Count, the Young Executive). Jack's realization that she is capable of love--and that she really loved Judge Irwin--helps him put aside his cynicism at the end of the novel.
- Willie's main political enemy within the state's Democratic Party, and governor before Willie. After Willie crushes him in the gubernatorial election, MacMurfee continues to control the Fourth District, from which he plots ways to claw his way back into power.
- The man whom Jack believes to be his father for most of the book, before learning his real father is Judge Irwin. After discovering his wife's affair with the judge, the "Scholarly Attorney" (as Jack characterizes him) leaves her. He moves to the state capital where he attempts to conduct a Christian ministry for the poor and the unfortunate.
- The "Young Executive," as Jack characterizes him; Jack's mother's husband for most of the novel.
Governor Joel Stanton
- Adam and Anne's father, governor of the state when Judge Irwin was Attorney General. Protects the judge after he takes the bribe to save his plantation.
- Willie Stark's Attorney General, an honorable man who resigns following the Byram White scandal.
- Governor of the state who sets Willie up as a dummy candidate to split the MacMurfee vote, and thereby enables Willie's entrance onto the political stage. When Willie learns how Harrison has treated him, he withdraws from the race and campaigns for MacMurfee, who wins the election. By the time Willie crushes MacMurfee in the next election, Harrison's days of political clout are over.
Mortimer L. Littlepaugh
- The man who preceded Judge Irwin as counsel for the American Electric Power Company in the early 1900s. When Judge Irwin took Littlepaugh's job as part of the bribe, Littlepaugh confronted Governor Stanton about the judge's illegal activity. When the governor protected the judge, Littlepaugh committed suicide.
Miss Lily Mae Littlepaugh
- Mortimer Littlepaugh's sister, an old spiritual medium who sells her brother's suicide note to Jack, giving him the proof he needs about Judge Irwin and the bribe.
- MacMurfee's most powerful supporter, a wealthy businessman. Willie is forced to give Larson the building contract to the hospital so that Larson will call MacMurfee off about the Sibyl Frey controversy, and thereby preserve Willie's chance to go to the Senate.
- Jack's sexy first wife, whom he leaves when he begins to perceive her as a person rather than simply as a machine for gratifying his desires.
Byram B. White
- The State Auditor during Willie's first term as governor. His acceptance of graft money propels a scandal that eventually leads to an impeachment attempt against Willie. Willie protects White and blackmails his enemies into submission, a decision which leads to his estrangement from Lucy and the resignation of Hugh Miller.
- A slimy MacMurfee employee who tries to bribe Adam Stanton into giving the hospital contract to Gummy Larson.
- A young girl who accuses Tom Stark of having gotten her pregnant; Tom alleges that Sibyl has slept with so many men, she could not possibly know he was the father of her child.
- Sibyl Frey's father, who threatens Willie with a paternity suit. (He is being used by MacMurfee.)
- The brother of Jack's grandmother. During the middle of the nineteenth century, Cass had an affair with Annabelle Trice, the wife of his friend Duncan. After Duncan's suicide, Annabelle sold a slave, Phebe; Cass tried to track down Phebe, but failed. He became an abolitionist, but fought in the Confederate Army during the Civil War, during which he was killed. Jack tries to use his papers as the basis of his Ph.D. dissertation, but walked away from the project when he was unable to understand Cass Mastern's motivations.
- Cass Mastern's wealthy brother.
- Cass Mastern's lover, the wife of Duncan Trice. When the slave Phebe brings her Duncan's wedding ring following his suicide, Annabelle says that she cannot bear the way Phebe looked at her, and sells her.
- Cass Mastern's hedonistic friend in Lexington, Annabelle Trice's husband. When he learns that Cass has had an affair with Annabelle, Duncan takes off his wedding ring and shoots himself.
- The slave who brings Annabelle Trice her husband's wedding ring following his suicide. As a result, Annabelle sells her.