Henry Martin opened the scoring with a close-range header on 12 minutes but Dong-Gyeong Lee pulled South Korea level eight minutes later with a shot from just outside the box.
Luis Romo re-established Mexico's lead on 29 minutes before a Francisco Cordova penalty six minutes before half-time, awarded after a foul by South Korea's Kang Yoon-Sung on Uriel Antuna, put Raul Gutierrez's side 3-1 ahead.
South Korea came out with intent in the second half and reduced the deficit on 51 minutes when Lee got his second after being set up by Kim Jin-Ya.
However, Mexico made it 4-2 just three minutes later as Martin doubled his tally.
Cordova then got his second on 63 minutes to net Mexico's fifth before Eduardo Aguirre added a sixth on 84 minutes.
Hwang Ui-Jo headed in a consolation goal in stoppage time for South Korea.
Mexico will face Brazil in Kashima on Tuesday for their semifinal in a repeat of the 2012 final.