The 18-year-old inked his name in Albion’s history books with a brace before the break to edge his side towards their first continental berth.
Mohamed Elyounoussi made things interesting when he headed home to narrow the deficit in the second half, which also saw VAR chalk off a potential Saints equaliser before Pascal Gross put the game away.
Sixth-placed Brighton, who have a game in hand, could still finish level on points with Aston Villa below them, but their final-day opponents would have to overcome a near-impossible goal difference to finish above the Seagulls and clinch the Europa League spot.
The big occasion was marked by the appearance of a band from North Carolina State University who lifted nervous spirits in the sold-out stands with a rousing rendition of Sweet Caroline from trumpets and sousaphones.
Kaoru Mitoma sent a shot just wide of the right post in the eighth minute after a slightly shaky start for the hosts, who saw Pervis Estupinan’s attempt from distance easily handled by Saints goalkeeper Alex McCarthy.
Closer still was Mitoma’s second opportunity, a lucky break when Jan Bednarek stumbled to set him free inside the six-yard box but the Seagull squandered the golden chance as he pinged it low off the post.
Soon, however, Brighton took the lead through 18-year-old Ferguson, who collected Alexis Mac Allister’s pass at the edge of the area and fired home his fifth Premier League goal of the campaign with a low strike that went straight through McCarthy.
Ferguson’s second was the result of some fine work from Mitoma to challenge Romeo Lavia for the ball at the halfway line, with officials dismissing the midfielder’s protests as he tumbled to the ground.
Mitoma scrambled down the left before teeing up the onrushing Irishman, who took his tally to 10 across all competitions this season with a finish in the bottom left corner.
The Japan international nearly added another in stoppage time but the two-goal advantage stood at the break.
Gross rolled a weak effort at Alex McCarthy to start the second half, which started more brightly for the visitors as they increasingly encroached on Brighton’s territory.
And they pulled one back in the 58th minute when Elyounoussi headed James Ward-Prowse’s corner.
Suddenly, it seemed like everything was unravelling for Albion.
Ward-Prowse played through Theo Walcott who lifted the ball over Jason Steele, but the home support released a collective breath after the potential equaliser was chalked off after a VAR check.
Moments later Gross extended the hosts’ advantage, picking up the ball – which had bounced in his direction from an aerial challenge – and took his time before firing in his 26th Premier League goal for Brighton, drawing him level with Glenn Murray and Neal Maupay for the club record.
Brighton, and an increasingly animated Roberto De Zerbi, wanted more and had late chances through Mac Allister and Deniz Undav, but no Seagull was rueing missed chances when the final whistle blew after a nervy seven minutes of stoppage time.