Former Spurs midfielder Mason, the Premier League’s youngest head coach at the age of 29, succeeded Jose Mourinho after the Portuguese was sacked on Monday.
He made a winning start to his tenure after Gareth Bale curled in a superb equaliser and then a late VAR intervention saw Son convert after a Moussa Djenepo foul on Harry Winks was shown to be just inside the penalty area.
Until Bale’s composed finish on the hour mark the hosts had been second best to a slick Southampton side who went in front when Danny Ings glanced in James Ward-Prowse’s corner.
With Harry Kane missing and still a doubt for Sunday’s Carabao Cup final, Spurs were flat for long periods and appeared to have been affected by their tumultuous few days, with Mourinho departing fewer than 24 hours after the club announced they were to join a new European Super League.
Chairman Daniel Levy has since expressed regret over the “anxiety and upset” caused by the proposal, which has now collapsed following widespread condemnation.
Signs of that backlash were evident prior to kick-off as groups of Spurs supporters gathered outside their club’s stadium calling for Levy and owners the ENIC group to leave the club.
Despite the unrest and an unsettling start to the week, this result moves Tottenham up to sixth – two points outside the top four – while the Saints remain 14th in the table.