With the game heading to a draw, Japan winger Mitoma produced a great piece of control before firing in from close range to stun the Reds.
Jurgen Klopp’s side, who were beaten 3-0 at the Seagulls two weeks ago, took the lead midway through the first half when Harvey Elliott slotted into the bottom corner from Mohamed Salah’s pass following a quick break.
However, Brighton equalised when Tariq Lamptey’s shot from distance deflected heavily off Lewis Dunk to fly into the back of the net.
It looked like a replay at Anfield would be needed to decide the tie before Mitoma stepped up to snatch another big result for Brighton.