The former Purbrook Park School pupil and Fratton Park season-ticket holder celebrated his goal for Frank Lampard’s men in front of the home fans at St Mary’s.
Mount struck on 24 minutes for Chelsea – and the Blues fan made an ear-cupping gesture towards the Southampton faithful as he wheeled off in celebration.
The 20-year-old previously revealed he turned down the chance to join Pompey’s arch-rivals Saints because of his roots to the Fratton Park outfit.
Mount’s goal at St Mary’s was his fourth in eight Premier League appearances this season.
His impressive form for Chelsea saw him rewarded with his first England appearances, coming on as a substitute in the Euro 2020 qualifier wins against Bulgaria and Kosovo last month.
Gareth Southgate also named Mount in the latest Three Lions squad last week ahead of qualifying matches away in the Czech Republic and Bulgaria.
Tammy Abraham, N’Golo Kante and substitute Michy Batshuayi got the other goals in Chelsea’s 4-1 win at St Mary’s.
Danny Ings struck for Southampton in the first half.