She was just a cute teenager in a sundress, sitting in a diner with her family and some family friends. Until her mother Deborah and Deborah’s partner, Todd Follansbee, settled the bill, and then suddenly Katie Ann Mayhew was on her way across the street to prepare to sing in Symphony Hall.

As surely every Islander knows by now, Katie was in Boston Tuesday night to sing in the finals of the Boston Pops High School Sing-off. One of three finalists, she would be singing her original audition song, Being Alive, from Stephen Sondheim’s musical Company.

A flock of supporters, from her fellow high school students to lifelong friends of her grandparents, had brought themselves from the Vineyard and other far-flung places to be present for something we all hoped (and some of us hubristically assumed) would be an historic event in the Mayhew family unparalleled since Governor Mayhew first stepped ashore on the Island.

Linda Berg was Katie’s vocal coach for years and had wanted to come to see her protégé in action, but circumstances made that impossible. Linda’s loss was my gain — suddenly there was a seat free in Jack and Betsey Mayhew’s car, and I grabbed the chance to go along. They, their daughter Lucy and I were on the second balcony, giving us a panoramic view of the spectacle on the stage. Even from the second balcony, the acoustics in Symphony Hall are pretty remarkable, and the hall itself somehow feels both majestic and intimate.

As it turned out, sitting directly next to me were a woman and her mother from Quincy who had shown up to support one of the other finalists. We all exchanged friendly smiles and handshakes, remembering how much Katie had lauded the friendly, noncompetitive feeling among the singers themselves. (I suspect we were not quite as noncompetitive, in our little tribal clutches up there in the balcony. But we were all very friendly.)

The Pops orchestra tuned and then quieted as conductor Keith Lockhart came out from a side door on the stage. The crowd applauded (as crowds do when conductors enter); Lockhart introduced himself and explained how the evening would unfold. First there would be a tribute to Leonard Bernstein, then the three finalists would sing and then there would be an intermission.

The second half of the concert would be dedicated to the Baseball Music Project, which is exactly what it sounds like: a collection of songs and orchestral pieces inspired by, or about, baseball. Then the winner (decided by three judges including Joyce Kulhawik) would be announced. Yeah, yeah, yeah, okay, we were all thinking, let’s get to the good stuff. The tribute to Bernstein was, you know, pretty fabulous, because after all — it was Bernstein. But still — let’s get to the good stuff, we were all thinking. And then it was time for the finalists. First was a lovely young dark-haired woman, Samara Oster, a senior from Milton Academy, dressed in a black robe. She sang Your Daddy’s Son from Ragtime, and she was — oh, dear — really wonderful. Intimidatingly good. A little over-expressive in her body language, I thought, desperately looking for the flaws, but she had a rich, confident voice, mature beyond her years.

When she finished, there was great applause. More than Keith Lockhart got, even! She’s good, I thought, but she’s not as good as Katie Ann Mayhew. I have a feeling I wasn’t the only one in the balcony thinking that. Then the Quincy competition came on stage. My neighbors in the balcony went wild. The Quincy song was not a solo, but a quintet from Les Misérables. The five high school friends and siblings had a fresh-faced, musical-theatre energy to them. They, like the first soloist, were dressed in black; they also had red scarves and belts. As well as singing and dressing in harmony, they moved in harmony, with simple but engaging choreography. The Mayhew family had been muttering about the Quincy Quintet since the semifinal round — the fear was that they were putting on more of a show and this might appeal to judges who were, after all, seeking entertainment for a Fourth of July evening.

And as Betsey (I think it was Betsey) pointed out, Les Miz is about the French Revolution, which made it somewhat topical to a celebration of the American Revolution. I watched them, wanting to be a harsh judge, wanting to find them not so hot, but the truth is, they were great. No individual in the group had a voice as good as Katie’s, but they made a strong, cohesive and extremely charismatic team. When they were finished, there was even greater applause (especially from my neighbors — and myself, because I was trying to be polite, and I wanted my neighbors to go bananas when the Vineyard girl came on. It was sort of enlightened self-interest for the greater good.)

And then after the tiniest pause, a slender, elegant waif in an off-white gown slipped onto the stage. She took the mike and with unassuming poise, she sang her heart out. It was enthralling, heart-stopping, gorgeous. She had better diction than anyone else, and her voice and words carried very clearly, even to those of us in the balcony; her body language was fluid, economical and yet so expressive. And her voice simply defied description, so I won’t waste words here pretending I am up to the task of describing it. I am of course biased but I truly believe the entire hall erupted into joyful hysteria when she finished. I believe it was the loudest applause I have ever heard in my life. I felt — I think we all felt — giddy, to have just witnessed such a soaringly heartfelt, confident performance. Do I sound sycophantic? Fine. I’ll sign up to be Katie’s groupie.

Intermission was interminable. On the one hand we were all on a high from having heard her sing, coupled with the happy relief that she had done such a smashing job. On the other hand, now there was nothing to do but wait. How on earth could we listen to an hour of freaking baseball songs when something so infinitely more interesting was just around the corner? Finally, after about three months, the lights flickered and we all rushed back into the hall to resume our seats. So now, I thought, we were going to sit through the Baseball Music Project? But instead, Keith Lockhart began the second half by announcing, “We have a winner. The judges have unanimously selected, from Martha’s Vine-” and I didn’t hear the rest because I and the Mayhews in the row in front of me were on our feet, screaming and clapping and cheering. Our Quincy neighbors were incredibly gracious (probably more gracious than I’d have managed to be in their place), congratulating me, as if I had anything to do with it — I assured them I didn’t, but that I would pass on their good wishes to Katie and her family.

There is a delicious kind of joy in being so absolutely happy for somebody else, somebody with whom you have little direct connection (I’ve only met Katie Ann a few times) but with whom you share many indirect connections — a little like rooting for a sports team. I thought at that moment of Katie Ann’s grandmother and mother, sitting down on the ground floor, and what this amazing moment must be like for them both. I got such a kick watching Jack, Betsey and Lucy from close range, exalting in the victory.

And then it was time to sit through the Baseball Music Project. I was still on such an adrenaline high, I could not bear to stand there with hand on heart to sing the National Anthem (that’s how that section of the concert kicked off), so I sneaked out into the hallway, called my husband and in-laws to shout, “She won!” into the phone, and then went back into the hall and somehow sat through the perfectly-nice-but-so-what rest of the concert.

After the concert I followed the throngs to the lobby, and with dozens of other well-wishers I waited as Katie was interviewed by radio, newspaper and television folks. She looked stunning and completely composed in her white gown, holding a bouquet of flowers with the graceful casualness of a veteran opera diva.

Once the interviews and countless hugs and congratulations and happy squeals (especially from and with her contemporaries) were over, Katie slipped into the dressing room to change back out of her evening gown. A group of two to three dozen of us, mostly from the Vineyard, made our way through the warm Boston evening to a hotel restaurant. In and amidst us all, as if she were nobody in particular, was a little slip of a girl in a sundress, with a black-and-white checked shirt thrown over it, walking in flip-flops and laughing with her friends.

Tonight she’ll be singing in front of an audience of half a million — not including the television audience of about seven million. Tuesday evening was radiant with joy and celebration. Tonight, I am sure, will be even more spectacular.