Three first-half goals were enough for Derby County to record their seventh win of the season and cement their place in the Championship playoffs this afternoon.
Craig Bryson put the Rams ahead after just three minutes when Theo Robinson sent the midfielder clear.
The lead was doubled before the 15-minute mark when Chris Maguire pounced after Robinson's shot was blocked by Henderson before Jamie Ward made it three.
The striker, who had an earlier effort saved well by Henderson, was fed by Jeff Hendrick before finishing coolly.
Portsmouth did grab a late goal ten minutes from time when James Pearce headed in from close range.
Nigel Clough's side move up to fifth and join a group of four teams on 24 points - six points behind leaders Southampton, while Pompey drop two places to 18th.
