Playing "mind games" with another person can be a pernicious undertaking, but that's not what John Lennon is singing about here. This song is inspired by a book he read called Mind Games: The Guide to Inner Space. Written by Robert Masters and Jean Houston, it explains how we can improve ourselves on various levels by playing tricks on our minds; the song is really about making yourself a better person.
This was the only single released from Lennon's Mind Games album, which was a modest success but nowhere near as popular as his Beatles output. This song has held up well, getting significant airplay on a variety of radio formats. In a 1998 interview with Uncut, Yoko Ono posited that the song may have been ahead of its time. "I think that people didn't quite get the message because this was again before its time," she said. "Now, people would understand it. I don't think in those days people knew that they were playing mind games anyway."
This started off as a song called "Make Love Not War," which had a strong antiwar sentiment. Lennon eventually abandoned that theme and wrote an entirely different lyric to the melody.