King, who has earned his fair share of off-field headlines down the years, is thriving at St Andrew's since his move in the summer of 2011.
The striker, having already clipped the outside of a post, won and then converted a first-half penalty to put Blues on level terms against Boro before his superbly-taken 81st-minute strike clinched three welcome points in a 3-2 triumph.
It took his goals tally for the season to 13, five behind the 18 he managed last season as he finished City's top goalscorer.
"Marlon's been fantastic for us," said Clark, who took the reins at Birmingham over the close season. "His winner is right in the corner of the goal, it's a terrific finish.
"Marlon's scored goals throughout his career, he's done it whichever club he's been at. Goalscorers are at a premium.
"That's why his performances on top of his goals have been terrific, but even if he's having an indifferent game then you try and keep him on the pitch because you know he can get you a goal out of nothing.
"He's done it with all different types of strike partners - Nikola (Zigic), Peter (Lovenkrands) and Leroy (Lita). He's been the catalyst for us."