The album was a commercial success, reaching number 4 on the Billboard 200, the band's highest charting album in the US. In the UK, the album peaked at number 6. The album is certified gold by the Recording Industry Association of America (RIAA) for 500,000 copies sold in the US. Following its release, Emerson, Lake & Palmer took an extended break from writing and recording.
To record the album, staff and equipment were brought in from Wally Heider Studios in Los Angeles, including a 24-track mobile recording unit and a 40-input console. Peter Granet, one of the engineers, called it "the finest recording experience I've ever had". The band used a QuadrophonicPA system on the tour, allowing a Quadrophonic mix of the album to be released on three 8-track cartridges. A four-channel sound LP, known as Quadradisc, was planned for release but it was scrapped due to engineering issues with master recording which prevented JVC, the manufacturer, from cutting a stable master to meet the format's specifications.
Most of the recordings on the album were first used for broadcast on the American rock music radio show, The King Biscuit Flower Hour. In 1999, the radio recordings were released on CD.
AllMusic gave the album a mixed retrospective review, saying that it "makes one realise how accomplished these musicians were, and how well they worked together when the going was good." They praised the set for including all but one song from Brain Salad Surgery, and particularly commended the performance of "Karn Evil 9" as being far superior to the studio rendition. However, they noted that unlike most live albums of the era, Welcome Back did not incorporate studio overdubs, limiting the band's ability to recreate moments from their albums and resulting in poor sound quality: "Even the most recent remastered editions could not fix the feedback, the occasionally leakages, the echo, the seeming distance – the listener often gets the impression of being seated in the upper mezzanine of an arena."