Adam Lallana gave the hosts an early lead in the 10th minute at the Amex Stadium and the Seagulls were good value for their advantage until a late fightback from the visitors undid their hard work.
Kelechi Iheanacho equalised for Leicester just past the hour mark and the game looked like it would be heading for a draw until Amartey headed home from a corner to score his first goal in over four years and condemn the home side to a third successive defeat.
Foxes show fight and character to come back
For large parts of the game on the South Coast, it looked like Leicester's patchy run of form would continue. With no wins in their last three, the early goal conceded set the tone for the Foxes who struggled to find their rhythm in the first-half.
Although Brendan Rodgers' side controlled the possession, they were struggling to create chances, however a moment of quality turned the game on its head.
The spark came from the East Midlander's midfield maestro Youri Tielemans who unleashed a no-look through ball to find Iheanacho who promptly finished to equalise.
A wave of energy had clearly been felt amongst the players on the pitch as the Foxes began to create more and more before a late header from Amartey - his first goal in 1540 days - demonstrated that whilst Leicester hadn't been at their best, they were still able to find a way to get back into the game and find the winning formula.
Leicester's most important win of the season?
There have been many impressive displays from Leicester this season, but in terms of significance, none were more important than the win on Saturday night.
With injuries plaguing the squad and a drop-off in form in recent times, there were fears amongst the fanbase that the Foxes form was all too familiar of last season and that a loss would see them face a tough fight to remain in the top four.
However, despite another unconvincing performance, Leicester were able to get over the line and gain further ground on the chasing pack below them. With many of the sides in and around them facing each other this weekend as well, this is a win that could be looked back on as an important moment in the season.
Iheanacho regaining confidence
For large parts of his Leicester City tenure, Kelechi Iheanacho has played the role of understudy to Jamie Vardy however with the 34-year-old struggling in-front of goal, it's the Nigerian who has stepped up to the plate.
His equaliser in Sussex on Saturday followed up his excellent volley at Burnley last Wednesday, with the 24-year-old capping off yet another excellent night on the pitch.
His strike on the South Coast takes his tally to seven goals this campaign - as many as his partner Vardy has from open play this season - and with each game that passes, Iheanacho becomes more and more confident.