Making their debut in the competition, the Irish were unable to cope with an attacking Polish outfit that dominated most of the opening match of the group.
After a goalless first half, a Jakub Blaszczykowski cross found Milik in the penalty area in the 51st minute and the forward blasted his left-foot shot through Craig Cathcart's legs and into the net.
It was Northern Ireland's first game at a major finals since they lost 3-0 to Brazil at the 1986 World Cup in Mexico, exactly 30 years ago to the day.
- Reuters