Critical response to Ordinary People has been mixed. Reviewers have found much to praise and much to criticize in Guest's novel. Many have found her characterization of Conrad Jarrett, the alienated teenager just released from a mental hospital, the most impressive aspect of the novel. Lore Dickstein writes in the New York Times Book Review that "Guest portrays Conrad not only as if she has lived with him on a daily basis—which I sense may be true—but as if she has gotten into his head. The dialogue Conrad has with himself, his psychiatrist, his friends, his family, all rings true with adolescent anxiety. This is the small, hard kernel of brilliance in the novel." Guest has acknowledged her fascination with adolescence; in an interview with former Detroit Free Press book editor Barbara Holliday, Guest says of adolescence, "It's a period of time ... where people are very vulnerable and often don't have the experience to draw on as far as human relationships go. At the same time they are making some pretty heavy decisions, not necessarily physical but psychological decisions about how they're going to relate to peopie and how they're going to shape their lives. It seems to me that if you don't have sane sensible people around you to help, there's great potential for making irrevocable mistakes."

Though Guest has been praised for her characterization of Conrad, some critics have found fault with the way Guest portrayed Beth, the distant, perfectionist mother in the story. Dorothea D. Braginsky writes in Psychology Today that "the mother's point of view, even though she is foremost in the men's lives, is barely articulated. We come to know her only in dialogue with her husband and her son, and through their portrayals of her. For some reason Guest has given her no voice, no platform for expression. We never discover what conflicts, fears and aspirations exist behind her cool, controlled facade." As critics have noted. Guest's narrative style. with its shifts between the perceptions of Calvin and Conrad Jarrett, leaves the reader with an incomplete perception of Beth. A further comment about Guest's style comes from Paddy Kitchen, writing in The Listener: "Judith Guest takes an 'ordinary' ... family in which the son, 17-year-old Conrad Jarret, has just returned home from a mental hospital, eight months after a suicide attempt. Her technique is to reveal information about Conrad and his parents, Calvin and Beth, in a colloquial, present-tense, piecemeal way—a method more often found in thrillers or adventure stories. She uses the technique extremely skillfully, with twist and turns that come like the proverbial unexpected cold buckets of water." Guest's "colloquial" technique has been praised by many critics for its realism. A Kirkus Reviews contributor writes, "where it does succeed, and succeed it does, is in communicating a sense of life both felt and experienced without ever trespassing beyond actuality. Ordinary People is an exceptionally real book."

Though many have praised her realism in portraying other characters, a number of critics have found Guest's portrayal of Dr. Berger, Conrad's psychologist, overly sentimental and unrealistic. They also fault her unwillingness to relinquish control over the way she tells the story, even though thematically she seems to argue against the desire to have too much control of anything. Several critics have also considered the ending to be much too neat. For example, Michael Wood writes in the New York Review of Books that Conrad "comes to accept his mother's apparent failure to forgive him for slashing his wrists, and his own failure to forgive her for not loving him more. It is true that she has now left his father, because he seemed to be cracking up under the strain of his concern for his son, but Conrad has learned 'that it is love, imperfect and unordered, that keeps them apart, even as it holds them somehow together.'" The critic explains: "It's an implausible conclusion, one of those secret happy ends that Hollywood weepies used to do so well: everyone dies, but they do it in the arms of the people they love, all error forgiven. Here the family is broken up, but everyone is on their way to emotional health, because they have understood their weaknesses. But then the whole novel is subtly implausible in this sense, not because one doesn't believe in the characters or in Conrad's recovery, but because problems just pop up, get neatly formulated, and vanish, as if they were performing a psychoanalytic morality play." While Melvin Maddocks praises Guest's realism, he too finds fault with the easy resolution of the novel: "the Furies in [Guest's] suburb are real, even if she seems to banish them with a spray of Airwick."

Despite the mixed critical reaction to Ordinary People, reviewers have found much to admire in the novel, and they have said that Second Heaven and Errands, two of Guest's succeeding books, prove that the success of Ordinary People was not a fluke. Moreover, Ordinary People was a resounding popular success, and critics have agreed that it is an excellent piece of popular fiction. It continues to be read and taught at both the high school and college level.