The original packaging, designed as a 12-page newspaper, claims the album to be a musical adaptation of an epic poem by fictional eight-year-old genius Gerald Bostock, though the lyrics were actually written by the band's frontman, Ian Anderson.

[11] To compensate for a lack of material, Anderson began work early each morning to prepare music for the rest of the band to learn during that day's session.

[20] The album prominently features flute, acoustic and electric guitars and Hammond organ, which had been used previously,[14] but the instrumentation includes harpsichord, glockenspiel, timpani, violin, lute, trumpet, saxophone, and a string section—all uncommon in the band's earlier blues-inspired rock.

[18] Anderson later said that the lyrics were partly derived from his own childhood experiences, though the overall theme was Bostock's attempt to make sense of life from his point of view.

[23] The mock newspaper, dated 7 January 1972, also includes the entire lyrics to "Thick as a Brick" (printed on page 7), which is presented as a poem written by Bostock,[18] whose disqualification from a poetry contest is the focus of the front-page story.

This article claims that although Bostock initially won the contest, the judges' decision was repealed after protests and threats concerning the offensive nature of the poem, along with the boy's suspected psychological instability.

[23] While some of the pieces were obviously silly, such as "Magistrate Fines himself", there was a lengthy story entitled "Do Not See Me Rabbit" about a pilot in the Battle of Britain being shot down by a Messerschmitt Bf 109.

[23] Most of the characters in the newspaper were members of the band, their management, road crew, or colleagues; for instance, recording engineer Robin Black played a local roller-skating champion.

[29] Following the album's release, the band set out on tour, playing the entire piece with some extra musical additions that extended performances to over an hour.

Chris Welch of Melody Maker praised the musicianship of the band and Anderson's flute playing, writing also that "the joke at the expense of a local newspaper wears thin rather rapidly, but should not detract from the obvious amount of thought and work that has gone into the production of Thick"; he described the music as a creative effort where "the ideas flow in super abundance" but that "needs time to absorb" and "heard out of context of their highly visual stage act ... does not have such immediate appeal".

[47] Tony Tyler in his review for New Musical Express generally appreciated the construction of the suites and the arrangements, but he had doubts about the album's possible success.

He called Thick as a Brick "Jethro Tull's own stand-or-fall epic after the lines of Tommy" and "an assault on the mediocrity and harshness of lower-middle-class existence in '70s Britain".

Whether or not Thick As A Brick is an isolated experiment, it is nice to know that someone in rock has ambitions beyond the four- or five-minute conventional track, and has the intelligence to carry out his intentions, in all their intricacy, with considerable grace.

[50] Village Voice critic Robert Christgau disliked the album, calling it "the usual shit" from the band: "rock (getting heavier), folk (getting feyer), classical (getting schlockier), flute (getting better because it has no choice)".

AllMusic wrote that "Jethro Tull's first LP-length epic is a masterpiece in the annals of progressive rock, and one of the few works of its kind that still holds up decades later.

"[36] Jordan Blum of PopMatters thinks that the album "paved the way for modern progressive rock" and "today, it represents not only a pinnacle achievement for Jethro Tull, but also a concrete example of just how adventurous and free artists used to be."

Record Collector's reviewer writes that "today, free from the irrelevant context of misdiagnosis and derision that dogged it on its original release, the album sounds like nothing less than an Olympian feat of composition and musicianship.

"[40] According to Modern Drummer reviewer Adam Budofsky, "that it remains so elevated in progressive rock fans' hearts and minds forty years later is a testament to its quality.

"[51] Paul Stump's History of Progressive Rock commented of the album, "The 'concept', a somewhat self-conscious and mannered lionization of individual genius ... against caricatured bourgeois Philistinism, scarcely sees the light of day through the thickets of Anderson's imagery, but this doesn't detract from what, in hindsight, is quite a respectable, if periodically water-treading, stab at advanced extensional technique.

A corrected replacement edition was issued later, with only a horizontal line beneath the "album duration" note on the disc label to identify it.

In the backstory Anderson created for the album, the now middle-aged Bostock came across an unpublished manuscript by one Ernest T. Parritt (1873–1928), entitled "Homo Britanicus Erraticus".

[79] Composer Richard Harvey has covered Thick as a Brick in the special compilation The Royal Philharmonic Orchestra Plays Prog Rock Classics.