Todorov had not netted since grabbing a hat-trick on the final day of Pompey's 2002-03 First Division Championship winning season at Bradford City.
The Bulgarian international made up for lost time by slotting home the only goal in the 56th minute.
Todorov was only on the pitch because Cameroon international Vincent Pericard had been forced to limp off with a thigh injury in only the sixth minute.
The Bulgarian showed nerves of steel to put Pompey into the lead after some great work from Lomana LuaLua.
LuaLua dribbled his way past two Albion defenders before pulling the ball back for Todorov to beat the onrushing Tomasz Kuszczak with a cheeky lob from six yards out.
The goal brought a rare moment of inspiration to a dire match, edged by the Baggies until half time.
Todorov's afternoon began when he was scythed down by a callous challenge from West Brom midfielder Ronnie Wallwork for which the Albion star was booked.
The revitalised striker could have been celebrating much earlier had he been able to connect with Laurent Robert's teasing right-wing cross 13 minutes in.
Richard Hughes then produced the tackle of the match to deny Pompey old boy Diomansy Kamara a minute later, after Kamara and Kanu had led a fine breakaway move.
Jamie Ashdown, the Portsmouth goalkeeper, was called into action for the first time in the 23rd minute when Nathan Ellington produced the first shot on target.
Ellington fired through the legs of Pompey defender Dejan Stefanovic but Ashdown got down well to his right-hand side to make a routine save.
That was the last time Ashdown, between the sticks after passing a late fitness test on a knee injury, was called into action.
Gary O'Neil saw his effort deflected by Paul Robinson into the arms of goalkeeper Kuszczak in the 34th minute.
The recalled Robert almost had Redknapp celebrating a minute later when he curled a 20-yard free-kick onto the roof of the Albion net.
That was the last meaningful action of a disappointing 45 minutes but Pompey, desperate to give their returning boss a first match victory, came out all guns blazing after their half-time team talk.
Todorov had a 52nd minute shot charged down by Albion defender Curtis Davies. Kuszczak then pulled off a superb double save to keep out LuaLua and Todorov in the 56th minute.
LuaLua scored a contender for goal of the season in Pompey's 3-1 defeat at Tottenham on Monday. The DR Congo man was at it again, unleashing a fierce right-foot volley from the edge of the area which Kuszczak managed to turn round the post and then beat Todorov to the rebound.
But former West Ham star Todorov was not to be denied and got his goal just seconds later.
Visiting boss Bryan Robson threw on substitute strikers Robert Earnshaw and Kevin Campbell for Kamara and the ineffective Jonathan Greening respectively. But neither player were able to put the Pompey defence under any serious pressure.
Redknapp was clearly unsure of what sort of response he would get from the home fans following his surprise return from arch-rivals Southampton.
The 58-year-old watched the first half from the director's box before hesitantly taking his seat in the home dug-out he occupied so spectacularly last time round for the second period.
Redknapp even strolled onto the pitch at the full-time whistle to applaud his players and the home fans who gave their seal of approval by chanting "There's only one Harry Redknapp" as the victorious players left the pitch.
MAN OF THE MATCH: Andy Griffin (Portsmouth) - The former Newcastle United man showed he has the bottle to help Pompey get out of jail with a wholehearted and disciplined performance.