Upon release, Wonder received mixed reviews from music critics, most of whom noted Mendes' vocal improvements, but were polarized over the album's production and songwriting, deeming them predictable and formulaic.

[20] Rolling Stone writer Jon Dolan defined Wonder as a compelling mixture of "youthful passion" and "coming-of-age agony", characterized its production as "grandiose" with backing choirs and dramatic drums, with sentimental lyrics inspired heavily by Camila Cabello.

He described it as a "headphone album, full of intriguing tones, lush textures, and unexpected twists", with "short and sweet" songs that lack the energy of his 2016 hit singles "Treat You Better" and "Mercy".

[28] Quinn Moreland of Pitchfork felt the album was "naive—and vaguely terrifying" while opining "it’s nice to see his cup overflow so bountifully, but the near-constant awe quickly grows tiresome."

He also noted that Wonder is Mendes' "most musically adventurous album" but that "every song is plagued by the same problem: Production that attempts to compensate for lyrical blandness by forcibly inserting drama."

[24] Leah Greenblatt, writing for Entertainment Weekly, asserted that the album sees Mendes "still discovering himself in real time", feeling free, expressing sexual tension and reflecting on early fame, and picked "Call My Friends", "Dream" and "Song for No One" as the best tracks.

[23] In a less favourable review, Helen Brown of The Independent wrote that Wonder chronicles "the struggles of the lonely touring musician missing his girl", and complimented the album's vintage-inspired sound, but deemed the songs unoriginal.

[22] Jon Caramanica of The New York Times complimented Mendes' "understanding [of] how to inflate his voice from whimper to peal," but felt that "his lyrics meander and stop short of true sentiment, and his rhythmic deliveries feel less cohesive" and that the record was "much less polished" in comparison to his previous two album releases.

[29] Writing for Stereogum, Chris DeVille named Wonder Mendes' "best (or at least most interesting)" album while complimenting his growth as a songwriter, although he feels that at times the record "falls victim to the same overly broad fairytale love-story clichés.

"[30] Wonder debuted at number one on the Canadian Album Chart for Billboard, earning the highest sales and digital song downloads and the second-highest on-demand streams for the week.