And he threw his potential new employers a Champions League lifeline.
Mane’s terrific treble coupled with Shane Long’s 100th goal in English football left boss Manuel Pellegrini regretting his call to leave out City’s big hitters.
Pellegrini dropped top-scorer Sergio Aguero to the bench while captain Vincent Kompany and key man Kevin De Bruyne were rested altogether ahead of Wednesday’s Champions League semi-final with Real Madrid.
The decision backfired as City’s second string were left humiliated by Mane and his Europa League-chasing pals.
Ronald Koeman’s Saints stormed into a 2-0 lead with two goals in the space of three first-half minutes.
The opener on 25 minutes was started by a raking pass from right-back Cuco Martina, which Dusan Tadic helped into the box with a delicious flick and the sliding Long toe-poked home from close range.
The goal saw Long chalk up double figures in a Premier League season for the first time, and City barely had time to catch their breath before Mane doubled the advantage.
Tadic showed City left-back Aleksandar Kolarov a clean pair of heels to race onto Victor Wanyama’s defence-splitting pass before cutting the ball back for Mane.