Agbonlahor's early effort - his fourth goal in four games - paved the way for Aston Villa's Carling Cup progress to halt a run of poor results against teams from lower divisions in the competition.
It was Villa's sixth straight win as they continue to build upon their best start to the season since 1998.
But Villa, five times winners of the League Cup, made hard work of their task against a Cardiff City side that played with no small measure of skill.
With five changes from the side which beat Portsmouth at the weekend, Villa made a perfect start when Agbonlahor nonchalantly flicked a John Carew cross past Peter Enckelman in his all pink outfit.
Surprisingly Villa, watched by their smallest crowd of the season, 22,527, failed to take full advantage of the situation and James Milner even missed a second-half penalty.
One of the pleasing features in Villa's latest win was the performance of teenager Fabian Delph who was preferred in midfield to the out of favour Nigel Reo-Coker.
He produced some skilful play against the Championship side which indicates a promising future at Villa Park.
Cardiff were certainly not over-awed despite trailing by the early goal. Gabor Gyepes went close to equalising, while Gavin Rae's swirling shot had Brad Guzan in all sorts of trouble.
Chris Burke proved to be a tricky customer, while Jay Boothroyd was a persistent threat as Villa survived several anxious moments as a result of some casual play.
The Welsh club kept plugging away as Villa appeared content to hang on to their slender lead.
Milner was always industrious but there was no one around to take advantage of his work.
But it was Milner who should have put the game out of Cardiff's reach in the 58th minute. Stiliyan Petrov was brought down by Gyepes, who was booked for the offence, in the penalty area.
Milner, however, blasted his powerful penalty kick against the post and in so doing raised Cardiff's hopes of an equaliser.
But Villa maintained their grip on the game and eased into the fourth round to the relief of manager Martin O'Neill.