The Irishman's 67 off 46 deliveries was the backbone of the Bears' 175 for nine, and Leicestershire's reply was hamstrung from the start as they were reduced to 29 for three before the late heroics of Tom Wells, with 51, raised faint hopes of an unlikely fightback.
In the end it did little more than prevent further damage to their net run-rate as Birmingham moved above them near the foot of the North Group standings.
The visitors got off to a solid start, with captain Varun Chopra and Porterfield one shy of 50 for the opening partnership when the skipper was bowled by Rob Taylor for 24.
The hosts got encouragement as captain Josh Cobb claimed the wicket of Jonathon Webb for five and, when Tim Ambrose followed, they were 69 for three.
But Laurie Evans gave Porterfield more solid support and, when the latter finally fell - caught off Cobb attempting a reverse sweep - Birmingham were 129 for four.
Very easy T20 Blast victory for the Bears, whose total was too chunky for Leicestershire to chase. pic.twitter.com/Mii4jrMkgL
— Sky Sports Cricket (@SkyCricket) June 1, 2014
Evans soon followed for 23 and Jeetan Patel's duck reduced the visitors to 134 for six, but Ateeq Javid ensured there was no late collapse as he made 27 off just 12 balls.
Set a target of 176, Leicestershire's reply started with an encouraging 11 off Keith Barker's slightly wayward opening over, but Chris Woakes shut them down in the second before removing Niall O'Brien cheaply.
Cobb - having already survived one appeal and been caught on a Barker no-ball - then followed for nine, a victim of Chris Wright, and, when Greg Smith was bowled by Boyd Rankin in the sixth over, Leicestershire were struggling.
It might have been even worse moments later but Ned Eckersley survived a run-out appeal which went to the third umpire with the score on 31.
Instead, he and Ramnaresh Sarwan, who made 37, were able to move the score on to 70 before Eckersley's number came up, stumped by Ambrose off Patel for 18.
That brought the 21-year-old Wells to the crease, and he reignited the contest with a string of boundaries, smashing 51 off 30 balls with three fours and a quartet of sixes before being bowled by Wright with the final ball of the match.
There was one scare, however, with Wells on 46 when Barker claimed what would have been a magnificent caught-and-bowled as the burly paceman dived to his right, but replays were inconclusive as to whether he got his fingers under the ball and Wells survived.