Millwall opened their Championship campaign with an away win at Middlesbrough thanks to Romain Esse's first goal for the Lions.
The 18-year-old broke the deadlock in the 79th minute when fellow substitute Aidomo Emakhu crossed from the left and Esse struck the ball into the top corner.
Sammy Silvera sent a fierce effort over the bar as the hosts, beaten play-off semi-finalists last season, sought an equaliser while Dael Fry failed to connect with a cross from the Australian.
Millwall's Zian Flemming was off-target with two early chances while former Boro man Duncan Watmore almost came back to haunt his old side but his shot was deflected over the bar.