Neil Diamond wrote this song. He had his first big hit earlier in 1966 with "Cherry, Cherry," which got the attention of Don Kirshner, who was looking for material for The Monkees. Kirshner was sold on "I'm A Believer," and as part of the deal, allowed Diamond to record the song as well.
Diamond's version was released on his 1967 album Just For You. The Monkees version benefited from exposure on their television series.
This was The Monkees second single, after "Last Train To Clarksville." It was released during the first season of their TV show.
The Monkees sang on this, but did not play any instruments. The producers used session musicians because they were not convinced The Monkees could play like a real band. This became a huge point of contention, as the group fought to play their own songs.