After a 2-2 draw in the first leg in Mexico, the Impact were on track for the last four until German Cano struck from the penalty spot in the 80th minute.
But Porter, introduced in the 85th minute, netted in additional time to level the game and the tie – seeing the Impact progress on the away-goals rule.
Frank Klopas' Impact side will play either Alajuelense or DC United in the last four, with the Costa Ricans having won the first leg 5-2.
Needing a goal to get back into the tie, Pachuca had to wait until there were just 10 minutes remaining.
With his team awarded a penalty, Cano stepped up and placed his effort to goalkeeper Evan Bush's right.
But, in the fifth minute of additional time, Porter struck.
Victor Cabrera played a well-weighted long pass, which Porter brought down with his chest to get past his marker.
Through on goal, he toe-poked an effort through Oscar Perez's legs to send his team through.
