"Us Against the World" has been received well by critics, citing it as one of the seminal moments on the album. The Chicago Tribune's Greg Kot commented on "Us Against the World", "U.F.O." and "Up in Flames" collectively, highlighting the songs as points where the band "milk these smaller, more intimate moments, the world-weary troubadour imploring a lover, God or both to lift up this blindfold (and) let me see again." In an article published by Q on 10 October 2011 entitled "First Impressions of... Coldplay's Mylo Xyloto" they wrote:
"Forever destined to be known as The One They Fucked Up At Glasto, Us Against The World begins with delayed patterns of guitar and a lone church organ, before stripping back to just Chris Martin and his acoustic. Its approach sums up what makes Mylo Xyloto ticks; its production is far from stripped-down, but, unlike the bloated X&Y and the do-you-know-what-it-is-yet-cos-we-don't mishmash of Viva La Vida, everything here is a servant to the melody. And, five tracks in, the melodies are some of their best; here, Chris Martin whispers "slow it down" as an orchestral swell builds around him. Just when you think it's going to embark on a Fix You-esque outro, it finishes, perfectly."
"Us Against The World" has been a frequently played song during the 2011 Festival Tour and the Mylo Xyloto Tour, and is statistically the most played non-single during the Mylo Xyloto album cycle. The song was first played at a friends and family gig at The Forum, Kentish Town, London on 31 May 2011. The song made its worldwide debut at the Rock im Park Festival, Nürnberg, Germany on 3 June 2011, and again at Rock am Ring the following day, along with other songs from the then-unannounced fifth studio album, Mylo Xyloto, including "Hurts Like Heaven", "Major Minus" and "Charlie Brown".
During the Mylo Xyloto Tour, "Us Against the World" has been played on every date of the Arena and Stadium phases of the tour. It had also been played on every date of the 2011 Festival Tour, but was cut from the setlist during the opening phase of the Mylo Xyloto Tour. Played as part of the main setlist during the Arena phase and as the encore opener during the Stadium phase, the song has been promoted heavily with performances on television and radio appearances, as well as appearing on the band's 2012 live album and film, Live 2012, which documents the Mylo Xyloto Tour.