Stafford shopping development The Grid posted on Facebook news of the long-awaited opening Friday.
The Katy location opened its doors to customers one hour earlier than planned.
By lunchtime, there were already dozens of cars lined up down the road, waiting in the drive-thru line.
The drive-thru customers were far from the first excited people on the scene. By 4:30 a.m. Friday, two people were already holding their place in line.
"I just love In-N-Out, they make good burgers. I like how they treat their employees," customer Damian Taylor said.
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Over the last several days, restaurant executives said folks could anticipate the opening soon.
On Tuesday, ABC13 learned there was a race to the finish for the two In-N-Out locations, with at least one other site waiting on a construction start date.
In-N-Out is anchoring The Grid, a 350,000-square-foot retail and restaurant development.
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In Katy, the restaurant joined the Y shops at Park West retail shops.
Employees at the Stafford location were spotted going over equipment and bringing in food items early Friday morning.