The Reds appeared to go a goal down in the first minute when Lomana Lua Lua beat Jerzy Dudek but his effort was ruled out for offside.
Liverpool went straight up the other end and almost took the lead through Milan Baros but Pompey goalkeeper Jamie Ashdown bravely saved at his feet. It was only a temporary reprive however, as Fernando Morientes gave the Reds the lead in the fourth minute following good work from Riise.
Morientes almost scored his second before Portsmouth scored an equaliser through Diomansy Kamara. It appeared the teams were going to reach the break all square but Luis Garcia scored his 12th goal of the season on the stroke of time after John Arne Riise had again put a dangerous cross into the box.
Portsmouth should have had an equaliser in the second half when the ball fell to Steve Stone, unmarked in the six-yard box, but he could only scuff at the ball and the danger was cleared.
Former Red Patrick Berger almost scored for Portsmouth but Dudek was able to palm his stinging shot away.
Liverpool had chances in the second half as well with Riise and Xabi Alonso both coming close to adding a third. In the end though, the Reds' first half display was enough to earn them their fifth away win of the season.