Katie Mayhew is one of three finalists in the Boston Pops High School Sing-Off. She sang with the 75-musician, full orchestra on Wednesday night, standing near conductor Keith Lockhart.

The 16-year-old singer will perform again with the Boston Pops on Tuesday, July 1 at Symphony Hall.

It has been an amazing month for the West Tisbury sophomore. Back in early May, Katherine Ann Mayhew — everyone calls her Katie — was one of 200 contestants from 120 communities vying for an opportunity to share their music on one of the nation’s most respected stages.

On Monday, May 19, she was named as one of the 22 contestants who would continue in the sing-off. In a night performance on Monday, June 2, she took her turn along with the others for a performance at Symphony Hall with a piano accompanist. From there the number was narrowed down to six semi-finalists, including her.

On Wednesday night she sang under chandeliers before a rapt audience of almost 2,500. She was dressed in a long burgundy gown and gold high-heeled

sandals. Her mother Deborah J. Mayhew and 30 of her Island friends and family sat below in the audience and watched.

She and her mother had purchased the dress over the Internet weeks before. Her blonde tousled hair was long enough to touch her shoulders. She sang her now-signature piece: Being Alive, a song from the 1970 Broadway musical Company, written by Stephen Sondheim. The song was once sung by actress and singer Bernadette Peters.

Katie’s friends and family had waited a long time to hear the young woman’s voice. It was a two-hour Pops concert that included a lot of familiar music, a tribute to Leonard Bernstein, and the musical piece To Lenny, written by composer and conductor John Williams. The Boston Pops performed Bernstein’s own work Overture to Candide.

It was then the time for three of the six semi-finalists; the previous three had performed on Tuesday night. Katie was the last of the high school soloists to sing. An intermission followed.

She said she and the other finalists had spent the evening in a chorus room below Symphony Hall, where they could not hear what was going on upstairs. The other two singers were Caroline Cronin, 16, from Norwell and Oliva Buckley, 17, from Wellesley. “We chatted,” Katie said. The three shared in the moment, full of anticipation.

“It was really, really amazing,” Katie said yesterday by telephone.

Yesterday, she and her mother were out celebrating by shopping for a dress for the July 1 concert.

“You know you don’t really notice the orchestra in the background, you are so focused on not messing up,” she said.

Deborah Mayhew said that during the intermission, they were approached by Tom Finneran, former speaker of the state House of Representatives and now a radio host. Mr. Finneran, along with others, came up to compliment the singer.

The evening of Pops music continued and finished with a performance by Broadway singer and actor Brian Stokes Mitchell. His last song before the encore, which ended the evening was The Impossible Dream, from Man of La Mancha.

Katie said she got a hug from Mr. Mitchell after the concert. “I was backstage getting my stuff to leave. He happened to be walking by. He came up and congratulated me on winning. He was really nice. It was kind of amazing. He gave me a hug,” which she said was one of the highlights of the evening.”

Afterwards the Mayhew family and friends went around the corner to a bar to celebrate for an hour. Katie said she spoke on the phone to her close friends and even exchanged e-mails.

She had high praise for her fellow contestants. “They were really nice. You know that they were really good, and did the best that they could do.”

Today is a bit of a reality check. Katie said she will be back in the high school finishing up her sophomore year. In having to go to the Tuesday morning rehearsal at Symphony Hall and Wednesday night’s performance, she has missed three days of school finals, which she makes up today. She said her teachers have been especially accommodating and helpful. “Teachers and students have come up to me in the hallways, to wish me well,” she said.

The finalists will perform with Keith Lockhart and the Boston Pops at Symphony Hall on Tuesday, July 1, at 8 p.m. The title of the Pops concert is Take Me Out To The Ball Game. Tickets are available from $19 to $87.

One of the finalists is Samara Oster, a 17-year-old student from Brookline attending Milton Academy, singing Your Daddy’s Son, from Ragtime. Another is a quintet from the Braintree and North Quincy high schools performing One Day More from Les Misérables. The singers are Adam Randall, 17, Meghan Ryan, 15 and Michael Ryan, 17, from the Braintree school and Brianna Shetler, 15, and Kirsten Shetler, 17, from the North Quincy school.

Samara and the quintet each performed with the Boston Pops at Symphony Hall on Tuesday night and were selected for the finals.

Only one of the three finalists will be chosen to join conductor Keith Lockhart and the Boston Pops when they perform on July 3 and 4 at the Charles River Esplanade. The Fourth of July performance will feature fireworks.

The relationship between the Vineyard and the Boston Pops doesn’t end with Katie Mayhew’s performance or performances in July.

The Boston Pops is scheduled to perform in Oak Bluffs at Ocean Park on Sunday, August 10. When asked whether Katie might perform with the Boston Pops when they come to the Island in August, a spokesperson for the Boston Pops said it was far too early to even talk about that as a possibility.