Albion had powered into a two-goal lead through Jason Koumas and Diomansy Kamara only for a Graham Alexander penalty and David Nugent to fire Preston back on to level terms.
All the more embarrassing for Albion was the fact that Preston had to play the final 49 minutes with ten men after the dismissal of Danny Dichio.
When Nugent fired them level in the 66th minute it looked like the Deepdale side could go on to secure their first win at The Hawthorns since 1973.
But, cometh the hour cometh the man, and Ellington proved to be Albion's hero only three minutes after being sent on.
However, his first goal owed a great deal to a huge error by the normally reliable Preston keeper Carlo Nash.
When Ellington whipped in a 75th minute free-kick from 20 yards it did not carry much pace and looked set to be comfortably dealt with by Nash.
But somehow the ball slipped through his hands and he could only watch in horror as it rolled over the line.
While Nash was to blame for that goal, there was nothing he could do with Ellington's second three minutes from time, when the former Wigan striker rose majestically to head home Paul Robinson's cross.
It was rough on Preston who had battled their way back from the worst possible start when Koumas' fifth goal in seven games and Kamara's 11th of the season had handed Albion the initiative.
But Alexander's spot-kick and Nugent's second goal in two games looked to have salvaged a point before Nash's howler.
Koumas took just six minutes to break the deadlock when he collected a cross field ball from Darren Carter and cut inside Alexander before curling his shot past the helpless Nash.
They doubled their advantage in the 25th minute with a quick break led by Carter.
Central defender Curtis Davies matched him pace for pace and when he found himself in an unaccustomed position on the left wing he whipped in a cross that Kamara drove him from 12 yards.
The goal looked set to signal the end of Preston's challenge but they were literally handed a route back into the game through Albion left-back Robinson.
He handled a cross from Matty Hill and referee Scott Mathieson had no hesitation in awarding a penalty with Alexander converting from the spot.
Albion were rattled at losing their two goal advantage but Preston looked to have lost their momentum four minutes before half-time when Dichio was dismissed for elbowing central defender Paul McShane.
All the dismissal did, however, was to galvanise Preston even more and they deservedly drew level in the 66th minute when Nugent turned home Chris Sedgwick's cross.
It looked enough to have earned Preston a point only for them to be undone by the heroics of Ellington.