The Warner Bros computer-animated movie took in $31.4m, earning a total of $183.1m. It beat two new releases - 3 Days to Kill and Pompeii - for the number one spot.
The Lego Movie features voices from actors including Will Ferrell, Liam Neeson, Morgan Freeman, Elizabeth Banks, Chris Pratt, Will Arnett, Alison Brie, Charlie Day and Nick Offerman.
Kevin Costner's action-thriller 3 Days to Kill debuted in second place with $12.3m, while Pompeii took third place with $10m.
Directed by Paul Ws Anderson, Pompeii is a disaster-adventure movie starring Kit Harington, Emily Browning, Carrie-Anne Moss and Kiefer Sutherland.
The RoboCop remake slipped from third place to fourth place with $9.2. The film stars Joel Kinnaman in the title role.
Both Ride Along and Frozen remained on the top ten