In a game of few clear-cut chances, Unai Emery's men were lacklustre throughout but their persistence paid off when the former Real Madrid winger pounced from eight yards to guide home an excellent lofted ball from David Villa.
Osasuna will feel hard done by as they appeared to have done enough to take a point, despite losing full-back Cesar Azpilicueta to two yellow cards in quick succession early in the second half.
However, their time-wasting antics ensured they were awarded little sympathy by the relieved Mestalla faithful.
In a disappointing first half, Valencia were tentative and showed glimpses of the fragile confidence that plagued them for much of last season.
So it was no great surprise when the game's first real chance fell to the visitors on eight minutes.
Jaroslav Plasil sprang the offside trap to race onto Javier Portillo's exquisite reverse pass and cut the ball back to Hector Font, but the forward could only blaze over when he surely would have scored had he kept the ball down.
However, the hosts did threaten to spring into life on a few occasions without ever delivering the final punch.
Joaquin and Manuel Fernandes worked some space from a corner to find Miguel alone on the edge of the 18-yard box but the full-back suffered a rush of blood and wasted the opportunity with a scuffed shot.
Mata tried his luck from 25 yards on the half-hour mark, his well-struck effort forcing Ricardo to dash across his goalmouth and he will have been relieved to see the ball drift wide of his left-hand upright.
Then Villa drilled a left-footed shot across the face of goal on 42 minutes with Joaquin unable to connect with an outstretched boot at the far post.
A minute later, Mata worked the visiting keeper with a left-footed strike from the edge of the 18-yard box.
In the second period, even half-chances were few and far between following the dismissal of Azpilicueta.
The full-back was given his marching orders for two fouls on Mata in the space of 10 minutes, the second for denying his opponent the opportunity to race into the box for a loose ball when both players were grounded in the corner.
From that point, the play became even more disjointed as the visitors played the clock as best they could, drawing cheap free-kicks out of the Valencia defenders and generally wasting time.
Los Che's chance to win the game appeared to have disappeared with four minutes remaining when Mata hit the base of the post with a fine strike from 15 yards, the ball bouncing back into the danger area but evading a trio of white shirts.
However, the Mestalla crowd didn't have too long to dwell on their apparent misfortune, Mata popping up less than 60 seconds later, latching on to Villa's pass and finishing with aplomb to secure the points.
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