[14] Writing for AllMusic, Stephen Thomas Erlewine wrote that "Millennium has no pretense of being anything other than an album for the moment, delivering more of everything that made Backstreet's Back a blockbuster.

"[1] Robert Christgau gave Millennium a two-star honorable mention, stating that the album is "softening it a little up for their younger demographic, sexing it up a little for their own peace of mind," specifically praising "I Want It That Way" and "Larger than Life.

"[15] Jim Farber of Entertainment Weekly gave the album a B−, stating that they have taken risks in their lyrics, as "Teen acts normally can’t acknowledge their romantic power.

[16] Arion Berger of Rolling Stone commented that the album was "prefabricated, too pretty, suspiciously well-choreographed", criticizing Nick Carter's straining vocals on "I Need You Tonight," calling "It's Gotta Be You" a rehash of their 1997 single "Everybody (Backstreet's Back)", and describing "The Perfect Fan" as bland.

However, he praised "Show Me the Meaning of Being Lonely," describing it as "digging its melodic claws into your skull on the first listen [...] it's the swooniest blending of the five vocalists' timbres to date, and mighty pretty besides".

[17] Writing for Spin, Joshua Clover criticized the opening track "Larger than Life," stating that it "boogies deftly and punks daftly [...] but huffs fame like glue", while praising other uptempo songs such as "I Want It That Way," "Don't Want You Back," "It's Gotta Be You," and "Spanish Eyes".

[19] He concluded by stating that while "the calendar flipping soundtrack" isn't Robbie Williams' song "Millennium" (1998), it 'smashes Silverchair's "Anthem for the Year 2000"' (1999), comparing the band more to Alanis Morissette than NSYNC.