She achieved this feat by appearing as a substitute in South Korea's 2-0 defeat against Colombia in Sydney.
Casey Phair surpassed the previous record held by Nigeria's Ifeanyi Chiejine, who was 16 years and 34 days old during the 1999 Women's World Cup.
The match also saw another record being broken, as the age gap between Phair and South Korean goalkeeper Kim Jung-mi was 22 years and 256 days, making it the largest age difference between teammates in Women's World Cup history according to FIFA.
Casey Phair is the first player of mixed heritage to be selected for a Korean World Cup squad, as reported by Reuters.
South Korea's coach, Colin Bell, stated that it was his responsibility to protect the young player from the intense media attention following her selection.
In the upcoming match, South Korea will face Morocco in Adelaide on Sunday.
Norman Whiteside holds the record for the youngest player to appear in the men's World Cup, representing Northern Ireland in 1982 at the age of 17 years and 41 days.