Hartley, 28, headed home from a 91st-minute corner to send the Pilgrims to Wembley for the first time in 20 years.
"I've never experienced anything like that, it's the biggest moment of my career so far," he told BBC Sport.
"I was sat in the dressing room and the boys were celebrating and I was just thinking 'what's happened?'"
Having drawn the first leg 2-2 at Fratton Park, the Pilgrims - who finished fifth in League two despite having led the league by five points in November - had almost all the chances in the second leg at Home Park.
But the game seemed to be heading for extra time until former Stevenage and Hartlepool defender Hartley came up with the winner in front of the Argyle end.
"I remember connecting with the ball and it hit the back of the boy's head and hit my head again and went in," he said. "Then I woke up and there was some big bald guy kissing me in the face.
"We set up perfectly to nullify Portsmouth's threat and I think we look like a real good outfit.
"I don't think Accrington or AFC Wimbledon will fancy playing us at Wembley."
Peter Hartley says scoring the injury-time winner for Plymouth Argyle in the League Two play-off semi-final against Portsmouth is his greatest moment.