The Society for Animal Protective Legislation (SAPL) decribed it as the end of the line for the plants.
A third plant, in Illinois, which was the last to operate, has also been subject to legal challenges, ending with a state law signed in the last week by the governor outlawing slaughter for human consumption.
However, SAPL says it is now critical that the US Congress act immediately to prevent American horses from being sent abroad for slaughter.
"Passage of the American Horse Slaughter Prevention Act (AHSPA) is the only way to ensure horses do not suffer even greater cruelty by being shipped to plants in Mexico and Canada," it said in a statement.
Chris Heyde, deputy legislative director for the society, said: "The American Veterinary Medical Association and other groups that worked to keep horse slaughter plants open have stated that it would be cruel to send horses to Mexico, and this is the one issue we both agree on."
Heyde, in talking of the Texas shutdown, said that, for four years, supporters of horse slaughter in Texas had tried to overturn a 1949 law that prohibits the sale, possession or transport of horsemeat for human consumption.
"Texas citizens have been loud and clear in their opposition to horse slaughter, and elected officials have listened. This is the second Texas legislative session to shoot down attempts by the slaughterhouses to overturn a longstanding law against this brutal practice."
In recent weeks, one amendment was tacked on to an otherwise uncontroversial bill in the Texas Senate, but was removed when it reached the state's House of Representatives.
Pro-horse slaughter lobbyists only a few days ago were in search of other bills that could be amended to keep these plants open, but their efforts ceased when it was clear that they didn't have the votes, according to SAPL.
"Hundreds of thousands of Texans are breathing a sigh of relief that we've finally driven a stake through the dark heart of the horse slaughter industry," said Susan Hendrix, vice president of Texas Humane Legislation Network (THLN).
"Even with some of the most influential lobbyists in Austin working as hired guns for the horse slaughter plants, it was clear to the majority of legislators that they were trying to defend a practice that was truly indefensible," she added.
SAPL commends the hard work and dedication by Texans who led the fight in Austin, including THLN, T. Boone and Madeleine Pickens, Habitat for Horses and country music legend Willie Nelson.