The Saints striker flicked the ball beautifully over toby Alderweireld before shooting past Paulo Gazzaniga for his ninth goal in his past ten games.
Spurs were humble, with Jan Vertonghen missing their best likelihood from shut target the first half.
Mourinho, who was reserved late on, sees his team keep sixth within the table, whereas Southampton rise to twelfth.
Spurs striker Harry Kane was forced off with a leg injury just seconds once having a goal disallowed for offsides within the last half.
Shane Long went shut with a header with 5 minutes left, however, Southampton is currently undefeated in their last four outings.
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