As far as I am concerned Queen never used a mellotron keyboard in their records or live. Today I was listening to Smile and noticed that Earth has a mellotron part in the backround! link
2.28-2.53
It was probably played by the unknown session musician who did the hammond organ work, but then again, Brian could have played it too.
The "strings" sound closely to MkII's 3 Violins, but as the album was recorded 1969, the mellotron could have been M 300 (1968) as well.