"[3] Allmusic critic Bill Janovitz described the song as "a graceful country-folk ballad" with "a wistful melody" sung "over a sparse guitar strum.

[1] Big Star biographer Bruce Eaton described the song as "Chilton's paeon to new love, conveying an innocence that belied the dark clouds that were already close on the horizon.

"[2] Both Eaton and Chilton biographer Holly George-Warren noted that the "I'm in Love with a Girl" echoes the sentiment of an earlier Big Star song, "Thirteen", which also had a folk music-like melody and used acoustic guitar.

[1] Music journalist John M. Borack considered "I'm in Love with a Girl" and the short song that precedes it on Radio City, "Morpho Too", to be "perfect closers to the album.

"[4] Classic Rock History critic Brian Kachejian rated it as Big Star's 6th best song, saying that the opening instrumental sounds like the Byrds or Bob Dylan, but that Chilton's vocal phrasing may have influenced Nirvana's Kurt Cobain.