Lawson frontman Andy Brown recently revealed that he wrote the track about his past relationship with the Saturdays singer.
King tweeted to her followers earlier today (May 27), urging them to download the track, which was released today.
"My favourite track of the moment - my good friends @LawsonOfficial! GO DOWNLOAD IT xxx," she wrote.
Brown had previously stated: "Yeah, it is about Mollie. She's my ex and we split up and I wrote quite a few songs about it and 'When She Was Mine' was one of them.
"But she knows and she's cool and she loves the song and we love The Sats so."
'When She Was Mine' has already cracked the UK iTunes top ten just a few hours after its release today (May 27).
Watch the music video for Lawson's 'When She Was Mine' below: