Pompey saw off League Two side Forest Green to reach the second round of the Carabao Cup.
John Mousinho's men fell behind to Tyrese Omotoye's striker, but Kusini Yengi quickly levelled.
And the forged ahead early in the second half through a fine finish from right-back Zak Swanson.
Yengi then wrapped up a 3-1 win from the penalty spot after Abu Kamara was tripped in the box.