Birmingham played with a refreshing attacking spirit. Jermaine Pennant, now firmly established in Steve Bruce's team, was their inspiration on a day when it was vital that the Midlands club emerged with three points.
On-loan Jiri Jarosik was on the mark as early as the fifth minute with Pennant adding a second in the 37th minute before providing an inch-perfect corner for Matt Upson to notch Birmingham's third goal after the break.
In the last couple of minutes substitutes Mikael Forssell, who converted a penalty, and David Dunn completed the rout.
Portsmouth, who are facing a Premier League inquiry into an alleged illegal approach for manager Harry Redknapp while he was still at Southampton, never recovered from Jarosik's early strike.
It was a massive set-back as they had opened in a positive style with Matt Taylor and Benjani Mwaruwari both testing the ability of Maik Taylor.
In a counter-attack Birmingham set the game alight with the opening goal from on-loan Jarosik but the honours go to the clever play of Mario Melchiot and Pennant.
They combined down the right for Melchiot to eventually chip a high ball to the far post where Jarosik was on hand to head home his seventh goal of the season.
Birmingham have been deeply perturbed by their slide into relegation trouble but this goal was a big fillip to their aspirations to move out of the danger area following some disastrous home performances.
Zimbabwe striker, Benjani, who was making a late appearance before jetting off to play in the African Cup of Nations was a big threat and Birmingham's notoriously shaky defence was far from happy on several occasions.
He set up Vincent Pericard only for the Pompey striker to see his shot go just wide to the frustration of Redknapp who responded by jumping out of his trackside seat in frustration.
Pedro Mendes, one of three former Tottenham players in the Portsmouth side, was flattened by Pennant's flying arm but this did not deter the talented Portuguese midfielder from dictating most of the Pompey attacks.
But just when Portsmouth appeared to be getting back into the game they fell for the sucker punch.
Emile Heskey headed a pass towards Chris Sutton and the former Celtic striker stepped over the ball to allow Pennant to gain possession and slot home.
Matt Taylor attempted a long-range, swerving shot which beat Maik Taylor but fortunately for Birmingham the ball flew wide.
It was that sort of game for Portsmouth. They have never scored at St Andrew's in the Premiership and that record continued to dog them despite some promising attacking play.
In sharp contrast it was Birmingham's day and unlike in the past they capitalised upon Portsmouth's shortcomings to carve out a well-merited win.
The game was put out of Portsmouth's reach in the 55th minute when Upson scored his first goal of the season following a corner conceded by Dejan Stefanovic.
Pennant swung over a high cross from the left and the on-rushing Upson powerfully headed home to seal the game. Benjani had a close-range shot brilliantly saved by Maik Taylor to emphasise it was not Portsmouth's day.
Noe Pamarot pulled down Forssell for an 88th minute penalty and to rub salt into the wounds Dunn raced clear to notch Birmingham's fifth and seal their runaway win.