Skipper Gordon Greer pulled a late goal back for the Seagulls before a crowd of 26,642, but Sami Hyypia’s men have now gone eight league games without a win.
Boro, unbeaten in their previous six meetings with Brighton in league and cup, came under early pressure and Kazenga LuaLua fired over before Gary Gardner had a tame shot held by keeper Dimitrios Konstantopoulos.
The Teesiders had a penalty appeal waved away when Adomah set up Tomlin, who was sent sprawling in a challenge with keeper David Stockdale.
Boro broke the deadlock in the eighth minute when, after a long ball by Greer was chased down, Adam Reach squared the ball for former Peterborough striker Tomlin who picked his spot from 15 yards.
The Seagulls Spanish defender Inigo Calderon put a deflected shot wide before Konstantopoulos, beaten only twice in his previous six appearances, showed alertness to hold a curling centre from Greer.
Brighton midfielder Joao Teixeira, on a season-long loan from Liverpool, shot wide for the Seagulls before later being forced off injured to be replaced by former Barnsley play-maker Paddy McCourt.
Reach had a shot blocked on the edge of the area after Ryan Fredericks was cautioned for the visitors for bringing down former Boro defender Joe Bennett.
Boro, enjoying a good spell before the interval, threatened when Lewis Dunk headed clear from Kike Garcia and then skipper Grant Leadbitter was off target from the edge of the area.
Both sides made changes at the break with Tomlin giving way to Jelle Vossen for Boro while the Seagulls introduced Chris O’Grady in place of Danny Holla.
LuaLua forced keeper Konstantopoulos to make a scrambling save low to his left after shooting left-footed under pressure from Leadbitter.
McCourt almost put the Seagulls level by firing just wide after beating three players in a dazzling run.
But Boro doubled their advantage on 53 minutes through Adomah who latched onto a cross from Daniel Ayala to hit his third goal of the season from close range, his initial effort having been parried by keeper David Stockdale.
LuaLua had a ball inside the area cleared as Brighton struggled for possession, and substitute O’Grady was later off target with a shot from just outside the area.
The Seagulls had a rare attempt on goal when Calderon’s deep cross was headed on by Bennett but giant Greek keeper Konstantopoulos made a comfortable save.
Bennett later crossed invitingly for Brighton but O’Grady failed to react in time at the far post.
Greer reduced the deficit for Brighton two minutes from time by heading in McCourt’s corner at the far post.