Repertoire.BMI.com credits it to Kris Kristofferson. So does the US Copyright Office. It has been recorded by Kristofferson, Johnny Cash, The Country Gentlemen, John Denver, Emmylou Harris, Waylon Jennings, The New Lost City Rambers, Peter Rowan, June Tabor, and several others.

I can't find any evidence that Ian & Sylvia ever recorded it.