Agbonlahor's strike partner Christian Benteke coolly slotted home a stoppage-time penalty to earn the hosts a battling 2-1 victory over their midlands rivals.
The forward duo were influential during a first-half display that will have impressed new manager Tim Sherwood, with Benteke heading on for Agbonlahor to clinically open the scoring after 22 minutes.
Saido Berahino got West Brom back on terms midway through the second period but Benteke had the final word after visiting goalkeeper Ben Foster brought down Matthew Lowton, ending a 12-match winless run and moving Villa three points clear of the relegation zone.
"I think it gives us confidence," Agbonlahor told BT Sport. "We've won a last-minute penalty after we haven't had luck all season. We had that luck today.
"I had a few chances, I'm happy to get on the scoresheet. But it was about team spirit - we believed that we'd get the three points today and we believe we can stay up.
"We're not going to look backwards now. We go into Saturday's game [against West Brom in the sixth round of the FA Cup] looking to win the game."
Agbonlahor thanked the Villa support for playing their part in a win that left his team-mates in high spirits.
"Any points are crucial at the moment," he added. "To get three points today - everyone's buzzing in that changing room.
"We've got to give credit to the fans. They were with us from the start, we conceded the equaliser and they stayed with us."
