The Australian was on fire following his solid performance for the Socceroos against Wales last week, scoring the equaliser and then having a hand in his sides other two goals.
Boro had to come from behind after Stephen McManus' fifth-minute own goal but soon kicked into gear when McDonald leveled after ten minutes when he swooped in to glance home Barry Robson's inswinging corner with his head.
The match was essentially wrapped up when new Boro signing Marvin Emnes grabbed his third league goal - and sixth in four matches in all competitions - when he received McDonald's 44th minute pass and finished past Steele to send the Teessiders to second place on England' second tier table, two points Southampton.
Tony Mowbray was delighted with the performance of the Australian and praised him for doing more than just getting his name on the scoresheet.
"I'm delighted for for Scott, it was a great flashing header," Mowbray told the club's official website. "I said to him that watching him play for Australia against Wales, I thought his link-up play was fantastic.
"Unfortunately we might lose him for a few games - on that evidence he's a stick-on to be in their side. I think we've found a way that helps Scott and helps our team but Scott will only be happy if he's scoring."