Holding a 3-2 advantage from the midweek first-leg Premiership promotion/relegation derby date, the Ports managed to get over the final challenge at the expense of neighbours Annagh.
The Ports had to wait until 87 minutes for the defining moment after clever attacking play between Billy Stedman and Stephen Teggart before the latter’s low cross was knocked home by the unfortunate Lee Upton, so impressive overall, for an own goal as he attempted to clear with Adam Salley ready for a tap-in.
With fewer than two miles dividing the clubs on the map, Annagh made the short journey still dreaming of completing a remarkable rise from Mid-Ulster level into the Premiership with promotion to the top flight.
Four-time Irish League champions Portadown, in contrast, were attempting to salvage a season of battling at the wrong end of the top table.
Portadown’s struggle for winning form cost Matthew Tipton his job as boss in January, with Paul Doolin returning in the New Year to the club he served with such success over the 1990s as a player.
The Ports closed out the regular season one place above relegated Warrenpoint with the challenge from Doolin of producing a final flurry.
The initial victory by 3-2 handed Portadown a slender lead on the pitch and home advantage for the Shamrock Park second leg, with a bumper crowd making the most of the historic move by club directors to offer free entry.
Events on the pitch - in contrast to the open and attacking first leg - proved competitive with minimal risk due to the high stakes.
Salley offered an early spark for the Ports but Annagh grabbed control when Peter Duffin found the net - only for the offside flag to rule out the goal.
Stephen Murray, just one of a number of former Portadown players in the Annagh squad, fired wide of the far post late in the first half off Ryan Harpur’s pass.
Annagh increased the pressure across the initial stages of the second half, with Michael Ruddy dropping back to cover on the line a looping, speculative effort by Aaron Rogers.
Annagh goalkeeper Eoin Hughes was alert to palm out Oisin Conaty’s cross before Josh Doherty could convert at the back post.
Hughes then dropped low to gather Salley’s long-range attempt before another dangerous teasing Conaty cross from the right into the danger area just evaded a sliding Portadown substitute Jack Smith.
Annagh had a second goal ruled out after Jordan Campbell steered home a corner-kick off Craig Taylor when referee Andrews signalled for a foul.
Then the 40-game season for Portadown came down to one magic moment to break spirited Annagh and protect the club’s Premiership position.
PORTADOWN: Barr, Hall (Finnegan), Ruddy, Salley, Conaty, McNally, Teggart, Beverland, Doherty (Smith), Stedman, Mashigo.
Subs: Burns, McCallum, Tipton, Anderson, Rutkowski.
ANNAGH UNITED: Hughes, McCullough, Upton, Campbell, Norton (Hunter), Harpur, Duffin (Carroll), Murray, Taylor, Rogers, Moffatt.
Subs (not used): Cusack, McConville, McKinney, Kerr, Malone.
Referee: Shane Andrews.