Daniel Ayala nodded the visitors in front in the 17th minute before Ashley Fletcher rounded off the performance with a quite astounding goal from all of 35 yards, deep into second-half stoppage time.
Albion enjoyed the opening stages of the game a little more, without asking too many questions of their rejuvenated visitors, and when the tables turned and the Teessiders decided to take the game to the home side, they made their own pressure count.
Boro got some early joy down the left-hand side and earned a corner, which towering defender Ayala headed home as he beat Sam Johnstone in the aerial battle.
From there Jonathan Woodgate’s youthful side had their tails up; they almost doubled their lead moments later when George Saville’s low strike was saved by the feet of Johnstone. Marcus Tavernier, shortly after, caught everyone out when his cross - meant for forward Fletcher - clipped the bar with the Baggies goalkeeper wrong-footed.
West Brom did look to reply after fashioning an attractive move of their own. Matt Phillips fed fellow winger Kyle Edwards, and he in turn picked out talisman Matheus Pereira at the back post.
Pereira’s first-time cross back across goal was headed just over the bar by the charging Gareth Barry, and Boro would take their deserved goal lead into the interval.
That scoreline, and the first-half performance of his side, prompted Slaven Bilic into a double change at half-time and he introduced both Charlie Austin and Conor Townsend.
Albion did indeed return with renewed vigour, and with Boro perhaps not expecting such immediate pressure. Jake Livermore forced Aynsley Pears, who had watched from afar in the first period, into a smart save and Barry slashed a volley wide.
As the half wore on, the toll of the festive schedule began to grab hold of both sets of players. Tired challenges were in regular supply and referee Geoff Eltringham was busy with his yellow card.
Austin thought he might have drawn the hosts level when he met a Pereira cross on the run, but Pears - in excellent form - saved low.
Fellow substitute Marvin Johnson could have killed the game off there and then as Boro broke away towards the other end as part of a frantic finish but his shot, while well placed, was straight at Johnstone.
Boro would win the game by two clear goals, though, when Fletcher caught Johnstone off his line from distance and found the back of the net.