Hagio drew inspiration for the series from the novels of Hermann Hesse, especially Demian (1919); the Bildungsroman genre; and the 1964 film Les amitiés particulières.

The Heart of Thomas was developed and published during a period of immense change and upheaval for shōjo manga as a medium, characterized by the emergence of new aesthetic styles and more narratively complex stories.

'"[16] That same year, Hagio and Takemiya watched the 1964 Jean Delannoy film Les amitiés particulières, which depicts a tragic romance between two boys in a French boarding school.

[21] Following the critical and commercial success of The Rose of Versailles at rival publisher Shueisha, Shūkan Shōjo Comic editor Junya Yamamoto [ja] asked Hagio to create a series of similar length and complexity, initially planned to be serialized over the course of two to three years.

[15][22] Hagio negotiated to allow serialization of The Heart of Thomas to continue for an additional month, stating that if the reception was still poor after that time, she would finish the story prematurely.

[23] In June 1974, the first tankōbon (collected edition) of Hagio's The Poe Clan was published: it sold out its initial print run of 30,000 copies in three days, an unprecedented sales volume at the time for a shōjo manga series that had not been adapted into an anime.

[33] By the Lake: The Summer of Fourteen-and-a-Half-Year-Old Erich (湖畔にて – エーリク十四と半分の年の夏, Kohan nite – Ēriku Jūyon to Hanbun no Toshi no Natsu) is a one-shot sequel to The Heart of Thomas.

[37] In The Heart of Thomas, Hagio develops important aspects of the principles of visual composition that have come to define the distinctive aesthetic of shōjo manga.

[38] She references jojōga (lyrical pictures) in particular,[38] a category of illustration which sought to create a mood of sad longing while also scrupulously depicting current trends in fashion.

[39] Seen significantly in The Heart of Thomas, these principles include characters externalizing their thoughts by associating freely or by doing so deliberately in a commentary; comic panels without borders; scenes displayed in slanting frames that overlap; visual metaphor; and backgrounds that arouse strong emotion.

[42] Kathryn Hemmann, a scholar of Japanese fiction and graphic novels,[43] interprets these visual metaphors to communicate the defenseless and guileless natures of the characters, and their pursuit of love unencumbered by sexuality.

[42] Folklorist Kanako Shiokawa comments on the use of emotive backgrounds as influencing the shōjo artistic convention of illustrations where blossoming flowers are crowded behind the large-eyed characters.

[44] Deborah Shamoon, a scholar of manga and animation,[45] downplays the focus on character and background design to consider the primacy of interior monologues in The Heart of Thomas, which are disconnected from speech balloons.

[39] The monologues are fragmented and scattered across the page, which Shamoon compares to poetry and the writing style of Nobuko Yoshiya,[39] and accompanied by images, motifs, and backgrounds that often extend beyond the edges of panels or overlap to form new compositions.

[51] Graphic designer and manga scholar Kaoru Tamura[52] compares the androgynous appearance of the boys of The Heart of Thomas to the titular character of Little Lord Fauntleroy by Frances Hodgson Burnett, which was translated into Japanese by Wakamatsu Shizuko.

[53] A notable exception to this artistic convention of depicting male characters with female attributes is Siegfried, who is drawn as masculine – taller, with hollow cheeks and oval eyes – even though his hair is unusually long.

[20] Mark McLelland, sociologist and cultural historian of Japan at the University of Wollongong, believed that Hagio depicted the primary characters of The November Gymnasium and The Heart of Thomas as male to free her readers from the same anxiety.

[58] Deborah Shamoon notes that although the gymnasium surroundings of The Heart of Thomas are represented with realism, the imaginative use of ghosts, angels, Biblical legends, and apparitions evokes a Gothic atmosphere.

[18] Supernatural objects and themes, in her view, represent not only the inner conflicts created by spiritual love but also reveal it as a force beyond ordinary rational understanding.

[1] Welker writes that Juli's character arc of being unable to love, befriending Erich, and leaving the school environment of Schlotterbach is one that is consistent with a "Bildungsroman paradigm.

"[64] Shamoon notes that unlike other manga works of the 1970s that feature male-male romance, The Heart of Thomas does not overtly depict sex; she argues that by depicting its characters maturing through spiritual and familial love rather than romantic and sexual love, The Heart of Thomas functions as a "transitional work" between "childish" shōjo narratives typical of the 1970s and earlier (such as Paris–Tokyo by Macoto Takahashi and Candy Candy by Kyoko Mizuki and Yumiko Igarashi, the latter of which Shamoon notes depicts "idealized romantic love in a heterosexual framework"), and shōjo manga from the mid-1970s and onward that targeted an older readership.

[19]Manga critic Aniwa Jun interprets Thomas' suicide not as a selfish act motivated by Juli's rejection, but as a "longing for super power, yearning for eternity, an affirmation and sublimation of life to a sacred level.

Randall notes how many of the stylistic hallmarks of the series, such as characters depicted with angel's wings or surrounded by flower petals, became standard visual tropes in the shōjo genre.

[70] In a separate review for Anime News Network, Rebecca Silverman similarly praises the "willowy" and dramatic 1970s-style artwork, particularly Hagio's use of collage imagery,[71] Writing for ComicsAlliance, David Brothers favorably compares the melodrama of the series to Chris Claremont's Uncanny X-Men and commends its character-driven drama.

[72] Publishers Weekly described the series' romance elements as "engaging but nearly ritualized," but praised Hagio's "clear art style and her internally dark tone.

In her survey of boys' love authors, sociologist Kazuko Suzuki found that The Heart of Thomas was listed as the second-most representative work in the genre, behind Kaze to Ki no Uta by Keiko Takemiya.

[78] Shamoon notes that much of the Western analysis of The Heart of Thomas examines the manga from the perspective of contemporary gay and lesbian identity, which she argues neglects the work's focus on spiritual love and homosociality in girls' culture.

[1] The Heart of Thomas was loosely adapted into the 1988 live-action film Summer Vacation 1999, directed by Shusuke Kaneko and written by Rio Kishida.

The adaption is considered a turning point for the company: it began staging plays with male actors exclusively, integrated shingeki (realist) elements into its productions, and changed its repertoire to focus primarily on tanbi (male-male romance) works.