Rangers right-back Patterson and Chelsea midfielder Gilmour, both 19, are the first graduates from the performance school programme to be called up to the senior international squad.
New Ross County manager Mackay, who left his role as Scottish FA performance director in November, insists he was always confident in the star quality of both players.
“Nathan is one we saw coming as a gangly 12-year-old who was about four stone wet through and five feet high,” recalls Mackay.
“But, my goodness, you could see the football potential of this youngster. That’s where you look at them and think ‘give them time’.
“Rangers gave him time and we certainly played him a lot with the Scotland youth teams on the way through. In the last year or so, he dipped into the under-21s and then Rangers have felt his progress has been such that he was involved heavily in the team which won the league this season.
“Billy is someone I’ve been impatient about for a number of years now. We took him to the Toulon tournament with Scotland under-21s when he was 17 because I had a firm belief this kid is something special.
“He ended up playing, becoming the breakthrough player at a tournament of that esteem, scoring a goal and captaining the team. It was only a matter of time but it’s terrific he and Nathan have been picked for the squad. That makes me more happy than you could ever know.”
Mackay fondly remembers his one game in interim charge of Scotland in 2017 against Netherlands at Pittodrie when he handed debuts to Callum McGregor, Ryan Christie and Ryan Jack who all went on to help the country qualify for the Euro 2020 finals.
But he feels the progress of the seven performance schools - Patterson attended Holyrood Secondary in Glasgow while Gilmour went to Grange Academy in Kilmarnock - give him most satisfaction when he looks back on his time with the Scottish FA.
“When I see a lot of the young players who played the night against Holland (when I was manager) and are now regulars, that makes me proud,” added Mackay.
“But going beyond that, when you see two kids like Gilmour and Patterson who came out of the performance schools, which are a huge success story privately and quietly in this country, it’s fantastic.
“They are bringing out players who get more touches of the ball than any other kids in Scotland. Over a period of years, that will flourish and come through.
“Players are now being seen and signed by foreign clubs at the age of 16 - that never happened in Scotland before.
“Money is coming back into the Scottish game because English clubs are paying for Scottish kids at 16.
“Also, the amount of 16-year-olds who are making first team debuts for Scottish clubs is catching fire.
“I’m really happy for the people at the Scottish FA - coaches like Brian McLaughlin, Billy Stark and Scot Gemmill - who have nurtured those players at international level.”