Muak Lek is the largest of the districts of Saraburi areawise, located in the east of the province. Neighboring districts are from the north clockwise Phattana Nikhom, Tha Luang and Lam Sonthi of Lopburi Province, Sikhio and Pak Chong of Nakhon Ratchasima Province, Mueang Nakhon Nayok of Nakhon Nayok Province, and Kaeng Khoi and Wang Muang of Saraburi.
The district is located within the hills of the Phetchabun Mountains, which separate central Thailand from the northeastern part (Isan). To the south of the district is the Khao Yai National Park.