After the struggles at the Euros and this Nations League campaign, Scotland needed to give the Hampden crowd something to feed off and had a massive chance to do that just four minutes in.
Ben Doak surged free wide on the left and teed up Andy Robertson to deliver a lovely cross to pick out McTominay completely unmarked in the box, but his header from eight yards out was straight at Diogo Costa in the Portugal goal.
It was the type of chance, against this level of opposition, you simply have to take.
Ronaldo, who broke Scottish hearts when the sides met in Lisbon last month with the late decisive goal in Portugal's 2-1 victory, had his first sight of goal, but his low effort from the edge of the box was comfortably saved by Gordon.
Ryan Christie stung the palms of Costa with a half-volley from the edge of the box, while at the other end Gordon was called into action to claw away a free-kick from Nuno Mendes.
The visitors were dominating the ball and Francisco Conceicao blew a decent chance when he ballooned his effort way over the bar after being teed up by Ronaldo.
Portugal continued to probe away at the Scottish defence without ever looking like stretching the hosts' rearguard to breaking point.
Substitute Lewis Morgan crossed for Che Adams to nod down into the path of McTominay, but the Napoli man could not apply the finishing touch.
A big chance came 15 minutes from time when Anthony Ralston sprung the Portugal offside trap, and with McTominay sprinting into the box in support, a pinpoint cross would surely have brought a goal, but the pass was heavy and the chance was lost.
Gordon produced a big save in the closing moments to deny Bruno Fernandes, and there was one more heart-stopping moment as debutant Nicky Devlin hurled himself in front of Rafael Leao's goal-bound effort to ensure the Scots held on for a point.