A big chance so far for the Azkals as captain Stephan Shrock found Chima Uzoka for a free header but went off at 36th minute.
Five minutes after the major attempt, Keo Sokpheng drew first blood for the Blue shirts as he scored from the close range, 1-0 for Cambodia.
And added time before halftime, the Young Azkals blew their two solid attempts but Cambodia didn't permitted those to penetrate.
54th minute of the Group A opener, Dylan De Brycker tried a long range but went off and on the 60th minute, Shrock took corner and located Gallantes but they wasted possession.
Earlier, Malaysia ended their quest against Myanmar into 1-1 while Group B opener Vietnam just routed Brunei, 6-0 at Binan.
They will face formidable Myanmar on Wednesday, 4PM at Rizal Memorial Stadium.