The Harris County Sheriff's Office said the shooting happened at about 1 p.m. just outside the Walmart on Highway 99 and Morton Road.
According to Sheriff Ed Gonzalez, the former "disgruntled" employee went into the store and was escorted out.
In an update, authorities said the former worker, identified as Armando Soto, got into a verbal disagreement with management before taking out a gun and walking out of the store.
Officials said he then shot toward the freeway and took off.
No injuries were reported, and the 21-year-old suspect has been placed into custody.
According to HCSO, it appears Soto was last employed in 2023.
Deputies are still investigating how many shots were fired and if Soto may have been intoxicated.
Watch live breaking news coverage from ABC13 on our 24/7 streaming news channel.