Cavel was appealing the earlier decision by the US Court of Appeals for the Seventh Circuit, which upheld the decision by the US District Court for the Northern District of Illinois reaffirming a state law preventing the slaughter of horses for human consumption.
"We are relieved and elated that the Supreme Court has decided this case does not warrant additional review," said Tracy Silverman, general counsel for the Animal Welfare Institute (AWI).
"We thank the Illinois attorney general's office for its efforts arguing to uphold the state law preventing the slaughter of horses for human consumption."
The AWI says its focus is now on the passage of the American Horse Slaughter Prevention Act, a federal bill to ban horse slaughter in the US and the transport of horses abroad for the same purpose.
It is pending in the US Congress.
Chris Heyde, AWI's deputy director of government and legal affairs, says its enactment is essential to ensure horses which would have been sent to Cavel are not exported for slaughter in Canada or Mexico.
"We hope other pro-horse slaughter organisations will now work with us to protect America's horses from this cruel and predatory industry," he said.
"The American public has made clear its desire to close this industry down."