Mlenga picked up a third gong, a bronze, in the mixed 4x400m relay to ensure SA ended the age-group showpiece top of the medals table with five gold, three silver and three bronze.
China was second and Cuba third, both with five golds, but fewer silvers.
SA's big push had come on Saturday when Breyton Poole took high jump gold while Sokwakhana Zazini and Zeney van der Walt won the boys and girls 400m hurdles.
Zazini won by nearly three seconds over his nearest rival, while Van der Walt put everything into her finish to edge Jamaican Sanique Walker by four-hundredths of a second after her rival, with a handsome lead, had stumbled over the final hurdle.
Lemao had taken SA's first gold on Wednesday in the 100m, with Mlenga second.
In the 200m on Sunday Mlenga was in front coming out of the turn and he held his advantage all the way to the line to ross in a 21.03sec season's best.
Lemao was second in a 21.12 personal best and Luis Brandner of Germany third in 21.23, also a personal best.
Mlenga later teamed up with Zazini, Van der Walt and Gontse Morake to finish third in the relay behind Brazil and Jamaica.
The US, Britain and Australia did not send teams to this event, also the final world under-18 championships, which began in 1999. Success there is not the same marker of success at senior level as the under-20 championships have been.
Of all the South Africans who won the country's 26 under-18 medals from 1999 to 2009, only high-jumper Jacque Freitag converted that into podium success at senior level.