The Yankee (or Hank Morgan)
- The central character and narrator of most of the book, his name is not given until close to the end. He is identified in various ways by the other characters in the book, most often by the title given to him by the common people of England, "The Boss." He is practical, business-minded, hard-working, determined, resourceful, intelligent, and possessed of an unbelievably good memory. He has a firm idea of right and wrong, and he is a staunch proponent of American democratic and capitalistic ideals. He believes in the power of technology and progress and detests narrow-minded adherence to superstition. He is a devout Protestant who values religious freedom and blames the Established Catholic Church for many of the ills of medieval society. He is a born leader, and the idea of inherited rank and strict social stratification disgusts him.
Clarence (or Amyas le Poulet)
- The Yankee's most trusted friend. He begins as a friendly but fairly ignorant page who takes a liking to the Yankee and offers him aid. He responds magnificently to the Yankee's tutelage and, by the end of the book, becomes thoroughly indoctrinated in the manners, speech, skills, and ideology of nineteenth-century America.
Sandy (or Alisande)
- A pretty but somewhat flighty damsel who comes to Arthur's court seeking assistance and becomes attached to the Yankee. The Yankee finds her terribly annoying at first, but she proves to be quite useful and pleasant. She is a product of her times in every way, believing fully in the righteousness of social stratification and the power of the supernatural, but she improves greatly through her contact with the Yankee and even teaches him a thing or two.
- A wise and gracious king. He tries his best, but he has a full share of the prejudices and superstitions of the day. The Yankee's opinion of his intellectual capacity varies, but he is capable of learning and of correcting his mistakes.
- A hack magician with the fatal flaw of actually believing in his own sleight of hand. Merlin is deceptive and petty and terribly vindictive; he would be an incredibly dangerous person if any of his tricks ever worked.
- The shining pinnacle of chivalry, often referred to as "The Invincible." He is noble and gracious and generally good in every way that can be expected from a man of his era, except for his tragic passion for Guenever.
- Arthur's beautiful queen. Everyone in the country knows of her indiscretions, except Arthur, of course.
Sir Sagramor le Desirous
- Your average haughty, block-headed knight. He angers quickly and holds a grudge.
- A knight who fancies himself a comedian. The sixth century finds him funny enough, but the Yankee simply cannot abide a particular joke of his.
Morgan Le Fay
- A vicious but beautiful and genteel queen. Arthur is her brother, but she hates him passionately.
- A cowardly and ineffective knight, who happens to be Arthur's foster brother and seneschal.
- A humble charcoal-burner, fairly representative of the poor-spirited peasants of the day.
- A prosperous blacksmith, extremely proud of his position as a self-made man.