When he was introduced by presenter Paul Rissman to the audience of 800 school pupils from the city and Ayrshire, he was greeted like a rock star with whooping, cheering and thunderous applause. Composers, however feted, are used to more restrained appreciation.
Of course, MacMillan's incentive to join the gathering was much more honourable than that, but the event was worth every decibel of the young people's enthusiasm. On stage under the baton of Pierre-Andre Valade were the orchestra that had commissioned the work being played, the concerto Veni, Veni Emmanuel, almost certainly the most successful work by the man introduced by Rissman as Scotland's greatest living composer, with more than 400 performances under its belt - a remarkable figure.
The soloist was Colin Currie, who must have given more of these performances than any other percussionist, including Dame Evelyn Glennie for whom it was written, and whose 1997 recording with the Ulster Orchestra likely found its way into more collections, not least because it cost £5.
The lunchtime concert was one of the SCO's Masterworks events from its Connect education and outreach arm, and in the second half we heard a full performance of the half-hour work, with Currie scampering about the stage between the huge array of tuned and untuned instruments. Before that, Rissman, with input from the soloist, the orchestra and the composer, took us through the piece, explaining its melodic origins in 15th-century plainchant, its rhythmic pulse derived from the human heartbeat, its utilisation of 12 tone and mainstream harmonic structures, its devotional inspiration and much else, with the help of superb projected graphics. It was captivating - and I thought I knew the work.
The Glasgow performance was one of three, alongside Edinburgh and Inverness, and I was delighted to hear how the young folk received it. Although I had wonderful music teachers in my younger days, I can recall nothing as inspirational as this event.