They accounted for Monivea 24-10 and Cill Dara 18-3 before securing their second bonus point win (20-10 against Cashel) on Saturday to clinch promotion.
That left Portadown clear winners of the round robin series, with 14 points and they will replace City of Derry in the league proper.
On Saturday, Cashel, the Munster League winners, ventured north with an outside chance of finishing second in the round robin series and gaining a shot at Waterpark in a play-off.
Credit to the Tipperary men, they put in a tenacious effort at Chambers Park, particularly in the forwards, but Portadown's defence was water-tight for the most part.
Cashel out-half Pat O'Connor kicked his side ahead with a tenth-minute penalty but three first half tries had Reid's charges leading 15-10 at the break.
Full-back Dave Whitten nabbed the first and wingers Nicky Jackson and Jonny Holden followed suit, with Cashel centre Colin Morrissey turned turnover ball into a well-taken try just before half-time.
Portadown threatened on many occasions in the second half and it became clear that Cashel's hopes were going to be dashed. They defended bravely with last-gasp tackles preventing Chris Banks and Stuart Douglas from adding to the hosts' tally.
But the Armagh men sealed the victory when a swift attack, involving backs and forwards, ended with hooker Ciaran O'Kane, a brother of captain Diarmuid, going over in the corner for the bonus point try.
Meanwhile, Galway outfit Monivea claimed second spot in the round robin series with a dramatic 20-17 win over Cill Dara at the Castle Grounds on Saturday. A late try from impressive back rower David Boyle was enough to give his side's their second win.
Trailing 17-13, the home side snatched victory from the jaws of defeat. In the fifth-minute of injury-time, Boyle managed to squirm his way over in the corner and Ian Murphy stepped up to add an excellent conversion.
It was heartbreak for Cill Dara who came into this game level on four points with Monivea.
Monivea will face Waterpark, the team that finished 15th in Division Three this season, in a winner-takes-all play-off (Ballinakill, Saturday, April 19, kick-off 2.30pm) to decide the 16th and final team that will compete in the third tier in the 2008/09 season.
Before that, Kevin Higgins' side have a Connacht Junior Cup final date with Galwegians at the Sportsground next Sunday.
The Connacht League winners are bidding to become the first club from the province to gain promotion via the round robin since Connemara did so in 2001.
2007/08 ALL-IRELAND LEAGUE JUNIOR CLUBS' ROUND ROBIN SERIES:
FULL RESULTS -
Saturday, March 22:
Cill Dara 10 Cashel 5; Monivea 10 Portadown 24
Saturday, March 29:
Portadown 18 Cill Dara 3; Cashel 8 Monivea 14
Saturday, April 5:
Monivea 20 Cill Dara 17; Portadown 20 Cashel 10
LIST OF CLUBS PROMOTED TO AIB LEAGUE FROM ROUND ROBIN SERIES:
2007/2008: Portadown (Ulster)
2006/2007: Naas (Leinster)
2005/2006: Rainey Old Boys (Ulster)
2004/2005: Nenagh Ormond (Munster)
2003/2004: Bruff (Munster)
2002/2003: Ards (Ulster)
2001/2002: Queen's University (Ulster) and Highfield (Munster)
2000/2001: Clonakilty (Munster) and Connemara (Connacht)
1999/2000: Naas (Leinster)
1998/1999: Barnhall (Leinster) and Thomond (Munster)
1997/1998: Banbridge (Ulster) and Midleton (Munster)
1996/1997: County Carlow (Leinster)
1995/1996: Suttonians (Leinster), Richmond (Munster), Ballynahinch (Ulster) and Creggs (Connacht)