The game had lacked chances before his introduction.
The visitors wasted their best chance when Che Adams volleyed over from the same spot in the first half.
Villa returned to winning ways in the Championship on Sunday afternoon with a narrow 1-0 victory over Birmingham at Villa Park.
Local lad Jack Grealish kept his place in the starting line-up after scoring his first goal since December in the defeat to Fulham on Easter Monday.
Villa boss Steve Bruce acknowledged the work Redknapp had done in a short space of time before going on to praise match-winner Agbonlahor.
It was not the start Redknapp hoped for after this week agreeing to oversee Birmingham's remaining three Championship matches following the departure of Gianfranco Zola.
"He was in the training ground at 8.20am this morning so that tells you something", Bruce continued.
Imagine if Gabby can turn up early at BMH every day? He's now got to win a contract for next year, which will be the first time in a long time.
The home side enjoyed more of the possession but struggled in the final third and failed to forge chances, Hogan flicking a header wide from Bacuna's cross in their only real attempt on goal.
Villa fans became increasingly irate at referee Simon Hooper's decision-making as the Wiltshire official let go a number of shirt-pulling incidents.
On the ball, their best attacks involved Lukas Jutkiewicz.
"Certainly he'll breed a bit of confidence and belief back into the players at Birmingham, he is wonderfully experienced". "But I didn't hesitate much when I got the call, I knew I wanted to do it. But off the bench came the threat they needed".
Bruce has stated that the home side's homegrown players were looking forward to locking horns with City.
Agbonlahor thundered into a challenge inside a minute, sparking contrasting reactions from both sets of supporters, and his bite and commitment sparked the game into life, though it also earned him a yellow card after a spat with Shotton.
"The other teams are going to win some games".
It was the 30-year-old's first strike since February 2016, however, leading Redknapp to deliver a wry take on events at Villa Park. It was his fifth goal in a Villa/Blues derby.
Redknapp's Birmingham looked to respond but Villa held their own excellently at the back, a sublime leaping header from Nathan Baker emphatically clearing danger on one occasion.
Villa is 12th in the Championship table at the moment, while Birmingham is only two points above the relegation zone.
It would have been met with a sigh of relief from Villa and Wigan fans alike as a point for Birmingham would have relegated the Latics.
Bruce said: "We've taken away the stigma that the players don't care, they're not interested and they're not bothered".