Its lead single, "Party", was released on July 7, 2015, and peaked atop the Gaon Digital Chart, further reaching number ten on the Japan Hot 100.

The group additionally embarked on a concert tour named Girls' Generation's Phantasia, which commenced on November 21, 2015, in Seoul and visited East and Southeast Asia.

[2] Echoing Zhang's viewpoint, Chester Chin from Malaysian newspaper The Star wrote that the album was a collection of bubblegum pop tracks.

[3] The record's opening track, "Lion Heart", is a soul pop-influenced bubblegum pop song which embraces a retro-styled sound while being instrumented by basslines and brass.

[5][6] Aside from the signature sound, Lion Heart also encompasses several other genres; "You Think" was characterized as an electropop and hip hop recording featuring trap beats and horns in its composition.

[2][7] "One Afternoon" draws influence from bossa nova and incorporates Spanish guitars,[2] while "Show Girls" portrays an electropop song originally recorded in Japanese for the group's 2014 greatest hits album, The Best.

[15] Subsequently, the group additionally embarked on a concert tour titled Girls' Generation's Phantasia, which kicked off on November 21, 2015, at the Olympic Gymnastics Arena in Seoul, and continued in visiting Japan, Thailand, Indonesia and Taiwan.

Slant Magazine's Anzhe Zhang wrote that the album was released to "quash" the suspicions that Girls' Generation was declining after the departure of member Jessica in September 2014.

"[2] Chester Chin, penning for Malaysian newspaper The Star, praised the release of singles "Party", "Lion Heart" and "You Think" as "a promising start."

Nevertheless, he disapproved of the rest of the album, dubbing it a "relatively tame offering" for "[traversing] way too quickly into filler territory" and criticizing the songs "Green Light" and "Paradise" for being too "generic.