Harrison took a previously written song (originally intended for Ringo), changed the lyrics, and recorded it with a little help from his friends. Ringo actually played drums on the song, and Paul McCartney (along with Linda McCartney and Denny Laine from Wings) sang backup vocals. As a result, this was the first song to feature the three ex-Beatles since 1970.
In the aftermath of Lennon's death, the feeling of sadness and nostalgia was palpable, and "All Those Years Ago" became a huge hit - reaching #2 on the charts (and only kept out of the top spot by "Bette Davis Eyes.") The video, featuring footage of Lennon, Harrison, and the rest of the Fab Four, fits the song quite well - it's more a celebration than a dirge.
After Harrison's 1982 LP Gone Troppo fell flat, he took a five year break from the business before his next album. He then remained sporadically active in the music industry until his untimely death from cancer in 2001.