Professor Godfrey St. Peter is an accomplished history professor in his middle age, living in the fictional town of Hamilton, Michigan. Having recently completed his eight-volume scholarly work, he is reluctant to move into the new home his family has built. He maintains his old attic study as a sanctuary for intellectual work and reflection, attempting to avoid the increasingly materialistic world around him.

Tom Outland is an orphan with a background in the American Southwest who becomes St. Peter's most brilliant student. He possesses an untainted mind and a deep passion for discovery, eventually inventing a valuable aviation engine. He dies in military service, leaving behind a lucrative patent and a towering legacy that the surviving characters struggle to honor.

Lillian St. Peter is the professor's wife, having met him while they were young students in Paris. She gracefully embraces the changes and comforts of middle age, fully inhabiting their new house and cultivating close relationships with her sons-in-law. Her growing social interests create a quiet but persistent distance between her and her husband.

Louie Marsellus is an engineer married to Rosamond St. Peter. Generous, enthusiastic, and wealthy, he successfully monetized the aviation patent left behind by Tom Outland. Louie frequently tries to win the approval of his father-in-law through lavish gifts and tributes to Tom, though his efforts sometimes overwhelm others.

Rosamond Marsellus is the older daughter of Professor and Lillian St. Peter. Previously engaged to Tom Outland, she inherited the lucrative patent for his aviation engine after his death. Now married to Louie Marsellus, she thoroughly enjoys her wealth and elevated social status, a shift that alienates her from her sister.

Kathleen McGregor is the younger daughter of the St. Peter family. Once a promising young artist, she is now married to journalist Scott McGregor. She harbors deep nostalgia for her childhood and the early days of Tom Outland's presence, actively resenting her older sister's displays of wealth.

Scott McGregor is a journalist who writes syndicated poetry and feel-good articles, married to Kathleen St. Peter. He feels overshadowed by the wealth of his brother-in-law and the mythic legacy of Tom Outland. Despite his somewhat cynical exterior, he maintains a genuinely fond relationship with his father-in-law.

Augusta is the longtime housekeeper and seamstress for the St. Peter family. Practical, grounded, and deeply religious, she shares the attic space with the professor to create a quiet companionship. She represents a sturdy, unpretentious reality that contrasts with the academic and materialistic concerns of the rest of the household.

Rodney Blake, known as Roddy, is a hardworking and unselfish man who befriends Tom Outland in the American Southwest. The two work together as cattle herders near the Blue Mesa, sharing an uncomplicated domestic life. He is intensely loyal and generous but struggles to understand the deeper historical value of their archaeological discoveries.

Doctor Robert Crane is a physics professor at the university in Hamilton. Despite having little in common socially with Professor St. Peter, the two men are allies in defending traditional academic values. He worked closely with Tom Outland in the laboratory but failed to secure his own financial stake in Tom's inventions.

Mrs. Crane is the assertive wife of Doctor Robert Crane. Concerned for her husband's failing health and lack of financial reward, she pushes for legal action to secure a portion of the profits from Tom Outland's engine. She blames Louie Marsellus for exploiting her husband's passive nature.

Horace Langtry is a younger history professor and St. Peter's professional rival. He embodies the shifting, highly political, and increasingly practical values of the modern university system that St. Peter actively disdains.

Henry Atkins is a castaway Englishman who falls on hard times and finds employment as a cook and valet for Tom and Rodney at their winter camp. He vastly improves their quality of life, contributing to their comfortable and masculine domestic arrangement.

Father Duchene is an educated priest and early mentor to Tom Outland. Possessing extensive knowledge of the Indigenous peoples of the Southwest, he provides vital context and interpretation for the artifacts discovered in the cliff city.

John Rapp is the foreman of the Sitwell Cattle Company. He assigns Tom and Rodney to the winter cabin near the Blue Mesa, warning them about the dangers of the area and strictly instructing them to focus on the cattle.