Lyric discussion by Erodox 

Cover art for Mister Kingdom lyrics by Electric Light Orchestra

For me the subject in the song has Passive suicidal ideation. The whole album seems to have a theme of general and specific dissatisfaction with life.
Two things about the song bring me to this conclusion.

1) The Chorus "Help me, such a lonely soul, in dreams, to leave behind the world". They want to leave the world but notes/qualifies it by saying (in dreams). 2) The Shakespearean reference "Oh to sleep, perchance to dream" has an established interpretation of longing for death in the hopes that there will be the ability to dream about pleasant things.

I "suspect" that Mister Kingdom in this sound would be God.

My Interpretation