The striker was born in the United States and was called up by the country at youth level, but has now made her debut for South Korea.
She came on in the 78th minute in Sydney, besting the previous record set by Nigeria's Ifeanyi Chiejine, who was 16 years and 34 days when she played at the 1999 World Cup.
Phair, born to an American father and Korean mother, is the first player of mixed descent to make the senior South Korean women's football squad.
South Korea's English coach Colin Bell called Phair the future after his side started their World Cup with a toothless defeat where they mustered only five shots all game. "She deserved to get the chance to play, she has trained really well, as good as anyone," said the 61-year-old coach.
"It is also a signal that that's the future, she is the future. We need strong, fast players with physicality."
Bell, who said on the eve of the match that he wanted to "safeguard" Phair from too much "hype", hinted that he could give the teenager more time in their next match against debutants Morocco.
"She gives us energy and power and that is where we need to step up and every player needs to step up in the next game," he said.
"I wanted to throw her on to give her that experience. At least we made history today," added Bell, referring to Phair's landmark.
South Korea went down meekly to Colombia, conceding a penalty in the first half and then a second before the break when goalkeeper Yoon Young-geul made a mess of a shot by Linda Caicedo.
"I am very disappointed with the result and performance," he said.
"The girls can play much better. At times they were too slow. We just did not create enough and our decision-making at times was not decisive enough."