Despite netting 32 goals for the under 17s and two for the under 19s last season Mark's form has dipped this term.
Having signed a professional deal in 2000, Mark would have been available on a free at the end of the season and Southampton have not stood in the way of his departure to Brentford.
Saints Under 19's coach Stewart Henderson said: "It is very disappointing in many ways as we thought he might be one for the future. He scored goals for fun at junior levels but struggled to have the same impact as he moved up through the ranks.
"He was perhaps not as focused as he was and as he needed to be and it was our decision to let him go. We would have released him anyway so at least this way we get a percentage if he is sold which we would not have done if we had released him.
"It may be that he will come good again. I hope he grasps this chance but he probably needs a fresh start. If he matures he might do a decent job there."
Southampton will be entitled to part of any subsequent transfer from Brentford.