This was the first song that Phil Collins ever wrote. He recalled to Prog magazine: "It was in D minor, the saddest of chords [grinning]. No, it really was in D minor. It was a very naïve but heartfelt ballad. This would have been in the late 60s when I was 15, 16 years old. Then I dried up until the mid-'70s."
The track was recorded by Phil Collins' band The Freehold in 1968 at Regent Sounds, London. Freehold comprised Phil Collins (vocals), Jeff Slater, Les Mannering and John Hunt.
Adam from MarylandI know in that 2016 interview Collins said the song was in D minor, but if you go listen to it it's actually not - it's in E dorian mode, which is a minor mode that shares notes with the key of D major. Can't blame the guy though for misremembering details about a song he wrote ~50 years ago and probably has never played since.
The lyrics for "Mary, Did You Know?" were written by Christian singer and comedian Mark Lowry, after his pastor asked him to write a Christmas musical for their church. Southern gospel musician Buddy Greene later added music to his words.