Ellis Harrison netted a double on his home debut after signing from fellow League One outfit Ipswich Town while Ben Close also found the back of the net for manager Kenny Jackett's side.
Birmingham made nine changes to the side that won 1-0 away to Brentford to open their Championship season, the most notable being 16-year old Jude Bellingham, who broke Trevor Francis' near 50-year-old record as the youngest player to feature in a competitive game.
Portsmouth also shuffled their lineup around after 1-0 defeat at the hands of Shrewsbury Town with Andy Cannon joining Harrison and Close in the starting XI in place of the suspended Ross McCrorie, the benched Gareth Evans and John Marquis, whose partner is about to give birth.
Story of the game
With Isaac Vassell linked to a move to fellow Championship side Cardiff City, the visitors looked a bit thin in terms of creativity. Bellingham was having a good go of it inside his opening 15 minutes and his first shot was deflected away after some neat interplay the sequence before.
Two minutes later, Dan Crowley was quick to react, but Pompey goalkeeper Craig MacGillivray snuffed out the danger by claiming the incoming cross. The home side were growing into the game and Marcus Harness let a shot go, speculative at best when he had options either side.
The hosts were in front on 34 minutes after Harrison headed past Blues keeper David Stockdale from a rebound off a corner. Defender Wes Harding tried to clear from his goal, but the referees had judged the ball to have already crossed the line only moments after Portsmouth hit the side netting.
Five minutes later, the lead was doubled. Ronan Curtis had his effort blocked, but the 22-year old Close saw the ball fall to him and his volley beat Stockdale low and to his left, the hosts well and truly in front as halftime beckoned.
The lively Curtis nearly added a third, his long-range drive just wide.
Pompey add a third to seal progression to second round
Inside of 10 minutes upon the restart, the game was put to bed.
Curtis headed wide, but Harrison was positioned at the far post to pick up the scraps, beating Stockdale with his second headed goal of the night, Portsmouth up 3-0.
He nearly completed his hat-trick after seeing Stockdale deny him from close range.
Lee Brown would be disappointed not to have made it four, his effort off of the rebound a miss that can only be characterized as shocking from five yards. The hosts finally had an attack to speak of, Steve Seddon setting up 19-year-old Odin Bailey, but the teenager blazed over, summing up Birmingham's night.
Blues captain Craig Gardner had perhaps the visitor's best chance in the 31st minute and he forced a terrific save out of MacGillivray off of a long-range shot from a free-kick as the Pompey keeper kept a clean sheet. The same two men were involved at the death, Gardner dipping his effort, MacGillivray saving once more to end a dream night.