Like other members of the genus Presbytis, this species has evolved to digest cellulose, and microbes in the gut extract nutrients from leaves.

[7] Due to the ongoing deforestation of the Leuser and Ulu Massen Ecosystems in Northern Sumatra, Thomas's langur is threatened with extinction.

As its habitat continues to erode due to human activity, it comes into conflict with people, moving into croplands where it becomes victim of the illegal pet trade and is seen as a pest by farmers, who cull them in large numbers with firearms.

Although protected by Indonesian Law, Thomas's langur is becoming habituated to tourists who visit Gunung Leuser National Park.

Though discouraged and prohibited, humans feed Thomas's langurs and other primates in the park causing negative consequences for wildlife and people.