Considered to be a spiritual sequel to the 1977 documentary Pumping Iron, the film gives access to the lives of the top seven bodybuilders in the sport, including Phil Heath, Kai Greene, Branch Warren, and Dennis Wolf.

[9] Ernest Hardy of The Village Voice gave the film a perfect score[10] and called it "a gorgeous meditation on age-old existential concerns".

[11] The Los Angeles Times called Generation Iron "an intimate portrait of bodybuilders" and praised the film as being "compelling and energetic".

Nicole Herrington of The New York Times gave the film 2.5 stars and wrote, "For a documentary about extreme discipline, the filmmakers lack restraint: the movie, about 20 minutes too long, undercuts much of its own momentum.

"[15] Nick Schager of The Dissolve expressed disdain for how the film "goes into absolutely no detail regarding the specifics of how [the bodybuilders] design their diets or their workout regimens".

[16] Film critic Tim Falkenberg of MXDWN praised the film for the same reasons, stating that "much in the way Ron Howard's Rush, released earlier this year, was about the competitors, not understanding Formula 1 competition, the documentary Generation Iron largely eschews educating its audience on the intricacies of its subject matter in favor of focusing on the competitors themselves".