Giant centre-back N'Gala scored two headers against Rochdale in Pompey's last home game and rose highest once more to guide a Ricky Holmes out-swinging corner past Brian Jensen after just four minutes.
Bury, who are now without a win in seven and sacked Kevin Blackwell on Monday, had their best chance of the half in the first minute but Shaun Harrad couldn't put away a Shaun Beeley cross.
Former Yeovil defender N'Gala almost went from hero to zero after 27 minutes but referee Carl Berry deemed his high and late lunge on debutant Danny Hylton only worthy of a yellow card.
The Shakers, currently in the hands of stand-in boss Ronnie Jepson, improved after the break and would have been level 10 minutes into the second half but for a Phil Smith wonder-save to deny a close-range Harrad header.
The struggling visitors dominated the second half but after Hylton and substitute Anton Forrester missed good chances, they almost conceded again when Jed Wallace hit the post late-on.