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.
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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 can anticipate the opening soon.
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On Tuesday, ABC13 learned there's a race to the finish for 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.
In Katy, the restaurant will join 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.