Cheick gets run-out as Nathan strike proves winner but Ronan sees red
The only goal came on the half hour mark with Ellington meeting Byron Lawrence's cross at the back post with a calm finish.
Ronan Murray saw red after a rare loss of control that saw him dismissed for what was judged as a head butt.
Luke Hyam twice headed against the bar at the end of the first-half and despite Brighton having the majority of possession, Blues held out for the win without too many alarms.
Cheick Kourouma, whose one year deal was finalised on Friday, made his first appearance as a registered Town player.