But Pearson also owed his first victory in six games in charge at St Mary's to a glaring miss by Leicester sub Matt Fryatt.
Trinidad & Tobago international John showed his predatory instinct with his 16th goal of the season to lift Saints five points clear of Leicester, who now occupy the last Championship relegation spot.
Leicester failed to deal with a long throw and, after Mario Licka clipped the ball back in, John capitalised on an error by Foxes defender Gareth McAuley to lift the ball over keeper Paul Henderson.
Leicester mounted a late rally to try and salvage a point and should have levelled through Fryatt deep into injury-time.
Steven Howard's nod down found Fryatt unmarked four yards out, but he let the Saints off the hook by screwing his shot wide.
Prior to John's goal Leicester had carved out the better chances in a scrappy game between two nervous teams.
Howard should have put the oxes ahead after just six minutes when Dudley Campbell's cross picked him out, but from six yards the striker slammed his shot against the net support.
Howard then had another sight of goal when Joe Mattock's cross found him between two defenders, but he was unable to direct his header.
It took Southampton 45 minutes to muster their first shot on target with Andrew Surman's effort comfortably saved by Henderson.
Saints had the first chance after the break when McAuley's under-powered header gifted the ball to David McGoldrick, but the young striker, making his first league start, lacked conviction and Henderson was able to save.
Leicester again wasted a chance to take the lead when Fryatt planted a weak header at Saints goalkeeper Michael Poke, also making his home debut, from six yards out.