Lee Dong-gyeong scored 10 minutes into the second half at their training base in Utah to give South Korea their second straight friendly win following a 5-0 victory over Trinidad and Tobago last week.
"We had some difficulties offensively early in the match but I think this match will be of huge help for us," said coach Hong Myung-bo.
"I think our players must have learned a great deal today by playing this type of team."
South Korea will face the Czech Republic, South Africa and co-hosts Mexico in Group A and begin their campaign against the Czechs in Guadalajara on June 11.
Captain Son Heung-min started on the bench and came on five minutes after Lee had given South Korea the lead with a curling, left-footed free-kick.
The 33-year-old Son, who has faced growing questions about his faltering form after scoring only twice for his club Los Angeles FC this season, scored twice against Trinidad and Tobago.
It took his international tally to 56 goals in 144 games, two short of Cha Bum-kun's South Korea record.
Defender Cho Wi-je made his debut as a second-half substitute.
Cho, who was playing in South Korea's second division until the start of this year, was called into the World Cup squad when Cho Yu-min was ruled out with an injury earlier this week.
"I think he played well stepping in for an injured player," Hong said.
"He helped the team tremendously."