A 28th-minute strike from midfielder Ondrej Duda was enough for the Polish side to seal their third straight victory of the group stages in Kiev.
Ivica Vrdoljak then missed a chance to double the visitors' lead from the penalty spot in the 51st minute, but it mattered little as Metalist attacking midfielder Jaja also failed from 12 yards 18 minutes later.
Henning Berg's Legia side top the group with nine points while Metalist are bottom with zero ahead of the return fixture next month.
Slovakian striker Duda's goal marked his second of the competition and came in stunning fashion as he cut in from the left flank and unleashed an unstoppable curling effort into the top left corner.
Legia were afforded the chance to make it 2-0 when Nigerian defender Ayila Yussuf handled in the area, but Vrdoljak was denied by a brilliant save down to his left by Metalist goalkeeper Olexandr Goryainov.
Vrdoljak then looked to have cost his team maximum points by conceding a spot-kick at the other end, but Jaja was thwarted by Dusan Kuciak as Legia held on.