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The Dutchman struck low past Tomas Kuszczak after 20 minutes to find a way through the division's tightest defence.
It was one of just two on-target efforts from Boro all match as Brighton dominated large spells of the action.
That was one of just a few occasions where Steele was called upon as Brighton were wasteful in front of goal.
Mackail-Smith was guilty of dragging a shot from a good position wide while Dean Hammond missed an even better headed opportunity just before the break.
Despite all of their possession, the Seagulls created few clear-cut chances in their search for a second-half equaliser, and when they did, Steele was on hand to save from Stephen Dobbie and Ashley Barnes.
Kazenga Lua Lua could have salvaged Brighton a point late on but his curling effort went narrowly over the crossbar.