The song has been a further top 20 success for Lemar; it was C-listed on BBC Radio 1, entering the UK Singles Chart at #61 after just two full days of download sales, and then climbing to #14 upon the release of the physical CD. The following week the single fell to #19 but Radio 1 moved the track up to their B-list. In its fourth week, "If She Knew" dropped two places to #21.
In a review from Digital Spy Alex Fletcher gave the song 3/5 and wrote, "For some reason he keeps his best vocals in the locker on 'If She Knew' and replaces his usual laid-back R&B with a futuristic pop sound that wouldn't sound out of place on a Kanye record. The results are slick and satisfactory, but this sounds more like passable album fodder than a storming comeback track."