La Cañada first-year Coach Alex Harrison, whose team won the Rio Hondo League championship and finished 14-6-5, also saw a defensive breakdown that led directly to Mountain View's first goal.
Mountain View's Ricardo Diaz got past a La Cañada defender and headed the ball past stunned La Cañada goalkeeper Graham Labran-Boyd in the 22nd minute.
"Just mistakes and letting kids get unmarked," said Harrison, whose team opened the season with a 1-1 nonleague deadlock against Mountain View (16-4-4) on Nov. 29. "[Labran-Boyd] couldn't turn in time to get to the play, but that's youth as well."
The Spartans, who opened the playoffs Thursday with a 3-0 home win against Hacienda Heights Wilson, couldn't mount many quality scoring chances against a Vikings team that finished second in the Mission Valley League before earning a first-round 4-3 overtime win against Garden Grove Santiago.
La Cañada's Armand Bagramyan, who finished the season with 23 goals, had his direct kick stopped by leaping Mountain View goalkeeper Christian Bucio in the 25th minute. In the 46th minute, Garrett Apel of La Cañada fired a 40-yard shot just wide of the right post.
Mountain View then closed out the scoring on a goal by Jesus Mariscal that landed just inside the right post past a frustrated Labran-Boyd, who finished with 10 saves.
The Spartans had the services of midfielder Zaki Khan for a limited amount of time. Harrison used him on several set pieces despite being hampered by a nagging ankle injury. They also were without Juan Carrasquero, who suffered an apparent knee injury against Wilson.
With the Spartans essentially without two of their top players, Harrison said they struggled to solve the Vikings.
"We gave away the middle of the field all day," Harrison said. "[Khan] is good at winning the ball and works for every loose ball.
"Not only that, he's 180 pounds. If he can't get the ball, he slows you down with his body. He has size in there."
Harrison said the Spartans have several things that they can build on leading into next season.
"Some positives," Harrison said. "The first goal was to win league and then win a [playoff] game. ... We have a young team that's growing."