The Scottish midfielder appears to have chosen The Den over Port Vale - where he spent ten games on loan earlier in the season - leaving Vale boss Lee Sinnott understandably disappointed.
"It has been a battle between us and Millwall." he told Vale's official website. "I have pushed as hard as I can for Marc's signature, but it looks like he has decided to join them.
"I have had a phone call from his agent, who told men it was 99.9% certain that Marc would join Millwall."
The 21-year-old as yet to play for City's first team and was placed on the transfer list earlier this season by manager Sven Goran Eriksson.