Albion defended resolutely throughout, restricting the hosts to few goal-scoring chances.
The Seagulls occasionally posed a threat, themselves, on the counter attack, with the offside flag denying Andi Zeqiri his first goal for the club.
After Cengiz Under also saw his goal ruled out for offside, it looked like the tie was heading for extra-time.
However, Brighton were dealt a cruel blow with just seconds remaining, when Iheanacho turned in Youri Tielemans' cross.
Scroll down and click through to see how journalism student Sam Cox rated Graham Potter's men out of ten.
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9. Percy Tau - 6
Still seemingly raw in terms of his adaptation to the English game. The South Africa international looked to use his pace to hurt City’s defence and came agonisingly close to scoring with 15 minutes of regular time remaining when he couldn’t quite reach Alzate’s cross for a certain goal.
10. Pascal Gross - 5
Added defensive solidarity that allowed Adam Lallana and Karbownik the option to go forward. However, with the ball at his feet, Gross was often wasteful and gifted possession back to the hosts on more than one occasion.
11. Andi Zeqiri - 7
One of Albion’s brightest sparks, Zeqiri was a willing runner throughout and never gave Leicester’s backline a moments peace. Thought he had his first goal in Albion colours when he strayed just offside before scooping the ball over Danny Ward.
12. Joel Veltman - 5
Was helpless to Leicester’s injury-time winner, but failed to prevent the hosts’ wave of late attacks.