Middlesbrough started Monday top of the table, two points and three places above Watford, who had failed to win their previous two matches.
However, Slavisa Jokanovic's men were the brighter team from the off at Vicarage Road and Deeney scored his 20th league goal of the season in the 37th minute when he controlled Adlene Guedioura's flighted cross and scored through the legs of Dimi Konstantopoulos from a tight angle.
Watford scored a crucial second in the 65th minute when Ighalo showed great control to shift the ball on to his left foot, before rifling his shot into the top right-hand corner from the edge of the box for his 15th league goal in the calendar year.
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