Composed by Ed Harcourt with additional writing by Ellis-Bextor, it is a chamber pop track which features percussion and string instruments.

"Runaway Daydreamer" received mostly positive reviews from music critics, who praised Ellis-Bextor's vocals and its sound; some denoted it as a highlight of Wanderlust.

The song was promoted with a music video directed by Sophie Muller, which largely shows Ellis-Bextor in a lounge room and walking through an abandoned pier.

[5] Davidf Farrell of PopMatters opined that the song and "Until the Stars Collide" set a "soaring, dreamy" atmosphere for Wanderlust.

[7] Considine highlighted "I could tie up the bed sheets and slide down the house / Be gone before the morning comes" as lyrics exemplificative of Ellis-Bextor's "artistic freedom" in Wanderlust.

[4] Reviewing the song for Loud and Quiet, Stuart Stubbs wrote that it "sees our protagonist pull herself out of squalor and onto a road of self discovery.

On behalf of Virgin Media, Matthew Horton opined that "Runaway Daydreamer" was the standout of Wanderlust, describing Ellis-Bextor's vocals in it as "prim and gorgeous".

[2] James Fyfe of The 405 echoed his view, praising Ellis-Bextor's vocal performance, as well as highlighting its sonority, despite observing it was not "musically adventurous".