Robertson and Liverpool lifted the Champions League title on Saturday as the Reds topped Tottenham 2-0 in the finale.
The Scottish fullback started and went all 90 minutes as an early Mohamed Salah finish from the penalty spot and a late Divock Origi clincher sealed the club's sixth Champions League title.
McTominay, a Manchester United academy product, says he takes pride in seeing one of his countrymen shine on the biggest stage, but his Scotland team-mate's achievement was the only positive from Saturday's result.
“I’m a Man United man through and through so I can’t lie it wasn’t easy, it was a tough watch,” McTominay told the Herald.
“But it was still good to see a fellow Scotsman on the pitch and beautiful to see what he’s gone through from 2012 to now.”
Robertson is set to join up with Scotland in the coming days, having been given a break to recover from Liverpool's Champions League run.
The defender has served as Scotland's captain, having earned 29 caps for the team since making his debut in 2014.
McTominay says his team-mate's achievement will serve as inspiration for the rest of the team going forward.
“It’s amazing for him and we’re relishing getting him back and hopefully kicking on in a Scotland jersey,” said the 22-year-old midfielder, who made 16 Premier League and three Champions League appearances for the Red Devils this campaign.
“If that’s not inspiration for the whole team right there I don’t know what is.
“He’s done incredibly well and that’s my aim - to be replicating something like that winning big trophies and hopefully playing in major tournaments in the future with Scotland.”