Border sheriff's have been informed about the deployment and the first Texas National Guard troops have taken positions in places such as towers used for monitoring, a deputy with the Hidalgo County Sheriff's office said.
Neither the Texas National Guard nor Texas Governor Rick Perry's office were immediately available for comment.
Perry visited guard troops at a training facility on Wednesday and told them: "Your mission is clear - to be a visible presence to deter criminal activity - and contribute additional eyes and ears to assist law enforcement and Border Patrol agents along the border."
Perry has said the deployment will cost at least $12 million a month, adding it was needed to secure the border because the attention of the U.S. Border Patrol was being diverted by a surge of children from Central America crossing into Texas.
Critics have questioned the spending, saying data shows the flow of children has slowed, more U.S. Border Patrol agents have been assigned to the region and the National Guard will not have the power to arrest, which raises questions about what the troops will do.
Perry, a possible Republican candidate in the 2016 presidential election, said he was forced into the move due to inaction by the Obama administration to secure the border which was allowing criminals to thrive.
Perry said the National Guard would help the state's surveillance and would deploy some of its assets, such as aircraft, to monitor the border.
Democratic lawmakers have said the deployment was more about politics than finding a practical solution to the problem.