Upon release, 30 Seconds to Mars received mostly positive reviews from music critics, who commended the album's lyrical content and the band's musicianship, which has been compared to the works of Pink Floyd, Tool, and Brian Eno.

[2] During this period, the band recorded demo tracks such as "Valhalla" and "Revolution", or "Jupiter" and "Hero", which later appeared on the debut album as "Fallen" and "Year Zero" respectively, but also "Buddha for Mary".

Thirty Seconds to Mars contacted Ezrin because they felt that he had the ability to help them achieve their own vision and create a distinct sound; moreover, the band's members grew up listening to his work with Pink Floyd, Kiss and Alice Cooper.

[8] The band and Ezrin chose an empty warehouse lot on 15,000 acres, starting an intense period of preproduction focused on fifty songs.

Jared Leto said that while recording in Wyoming, "there was an interesting dichotomy, a kind of contradiction with the technology and the organic world that we were surrounded [...] It's something that I think ended up in the music".

Thirty Seconds to Mars initially thought to exclude the track from the record since they were not satisfied with it, but then they decided to rework the song because there were people who had strong feelings about it.

[10] The style of the album combined progressive metal and space rock with influences and elements from new wave and electronica, utilizing programming and synthesizers.

Elizabeth Bromstein from Now magazine described it as a concoction of '80s electronics, industrial music and nu metal with a heavy dose of progressive rock.

[16] Jaan Uhelszki of Alternative Press felt that the album is made of "sterner stuff", with the band's prog-metal foundation "enhanced by an unexpectedly powerful sense of melody".

[9] He cited the human struggle as the most inspiring source for the band and explained that the record features "a definite desire for change, for renewal, for a new beginning and ultimately, for escape".

[19] Author Karin Lowachee commented that Leto, who wrote most of the lyrics, allows the listener to draw his or her own conclusions to the meanings of the songs.

[9] The album ends with the hidden track "The Struggle", whose lyrics were taken from the ancient Chinese military treatise The Art of War, attributed to Sun Tzu.

He said that the band "didn't want a song like that to overshadow what we are about" and especially after the September 11 attacks, they felt it did not fit thematically with the rest of the album, stating that "it took on new dimensions".

[24] The album's compact disc featured enhanced material developed by Little Lion Studios, including a promotional video titled "Capricorn" directed by Lawton Outlaw, and a behind the scenes footage edited by Ari Sandel.

It was broadcast worldwide through the online platform VyRT and included live playback and commentary of the record with the band, interactive discussion focused on the formation of Thirty Seconds to Mars, and exclusive acoustic performances.

[39] In early October, they were invited by MTV to join the Campus Invasion Tour, playing ten dates in Canada alongside I Mother Earth, Billy Talent, and Pepper Sands.

[45] Ryan Rayhill from Blender noticed "throbbing synths" and "exploding guitars", and called the album a "high-minded space opera of epic scope" narrating "tales of living in deep, black oblivion".

[11] Smiley Ben of BBC Collective described the overall sound as "certainly compelling", and praised the sonic variety, writing that the band "knowingly push[es] boundaries" producing "great music with an edge".

[18] AllMusic reviewer Jon O'Brien called the record a "highly ambitious space-themed concept album", and commended its "heavy, riff-laden" guitars, "soaring" vocals and sci-fi lyrics.

[50] Jeremy Gladstone of Kludge commented that the album reaches its peak with "Buddha for Mary", which he called a "virtual sonic achievement", but felt that the quality of the songs drops off at the halfway point, describing the entire project as "top-heavy".

[49] Canadian critic Karin Lowachee found the group's sound and vision reminiscent of their "concept-styled forbears" but "uniquely owned" for a contemporary audience, and wrote that the band "defies the trend by stepping out ahead of it and into the future", giving the listener something original to enjoy.

[16] Peter Relic of Rolling Stone gave a mixed response, stating that the album has its finer moments but felt that it is undone by Jared Leto's "baffling, pretentious poetry" and the "sanitized quality of the heavy guitars.

[63] In Greece, it entered the national albums chart in June 2011, in view of a band's concert held in Athens in July, reaching a peak of number 41.

[65] Jason D. Taylor of AllMusic considered the release of 30 Seconds to Mars a "daring new step in musical evolution" and noted that its impact left "vast corridors of sound open for the listener to meander down at will".

[56] 30 Seconds to Mars was named one of the best albums of 2002 by a number of publications, including CMJ New Music Report, Kludge, Melodic, and Metal Hammer.

[7] Artistdirect agreed and explained that 30 Seconds to Mars established the group as "fresh, new force", revealing a "multi-faceted outfit that thrived on creative exploration and escape".

deemed the record an "epic starting point" for the music that followed, and stated that it displayed Jared Leto's "vast, cinematic writing style".

He considered its music "undoubtedly the most aggressive they've created" and regarded lead single "Capricorn (A Brand New Name)" as the band's "most muscular moment".