Super-sub Ellington, who remains on the transfer list after asking for a move last month, had only been on the pitch six minutes when he made the decisive breakthrough in the 66th minute.
There appeared no danger when he collected the ball from Jason Koumas some 25 yards from goal but the former Wigan striker bent his shot around the previously unbeatable Neil Alexander.
The goal, Ellington's seventh of a stop-start season, earned in-form Albion their ninth win in their last 10 home games.
But it was somewhat rough on play-off chasing Cardiff who had more than held their own despite being without their suspended leading scorer Michael Chopra, as well as the banned Simon Walton and Glenn Loovens and the injured Darren Purse and Ricardo Scimeca.
With Alexander keeping Albion at bay with some stunning first-half saves, Cardiff, who had won four and drawn one of their previous five games, had enough chances to have taken the lead themselves before Ellington came off the bench in such dramatic fashion.
But they were out of luck in front of goal - a fact summed up in the 72nd minute when Dean Kiely tipped Steven Thompson's fierce goalbound drive over the bar to effectively end the City challenge.
While Alexander ensured that Cardiff got to half-time on level terms, City could justifiably have felt they deserved to be in the lead themselves after an entertaining first 45 minutes.
Alexander set his stall out as early as the third minute by tipping away Robert Koren's 20-yard drive before going on to frustrate the Slovenian international again six minutes before half-time.
He also denied Koumas, Kevin Phillips, who had a header disallowed for offside, and Diomansy Kamara as Albion peppered the make-shift Cardiff defence.
While Cardiff had problems in keeping Albion in check they carried as much of a threat themselves going forward - particularly through Peter Whittingham and Jason Byrne.
Thompson should really have converted Whittingham's sixth minute cross before Byrne was denied by a point-blank save from Kiely.
Whittingham caused all sorts of problems for Albion and saw his 33rd minute drive whistle just wide before Thompson pounced on a misplaced clearance from Kiely only for his shot to hit the bar.
Albion had another let-off in the 37th minute when Whittingham's corner was headed just over the bar by central defender Roger Johnson.