A major rewrite of the script was needed for this two-part episode as it was originally intended to be the series finale. However, Angela Lansbury was persuaded to continue the series with the promise of a reduced workload (hence the "bookend" episodes in the subsequent sixth and seventh seasons).
The title comes from the Wicked Queen summoning the spirit in the magic mirror in the fairy tale Snow White and the Seven Dwarfs (1937): "Mirror, mirror on the wall. Who's the fairest one of all."
"Mirror, Mirror on the Wall" was the third of four two-part storylines produced over the series' 12 seasons. The other three are: "The Murder of Sherlock Holmes" (S1, E1&2, 1984); "Death Stalks the Big Top" (S3, E1&2, 1986); and "Nan's Ghost" (S12, E6&7, 1995).