Vast quantities of marigold flowers are often discarded as waste at sacred places and temples after religious ceremonies in Thailand. This has motivated us to examine the utilization of waste marigold flowers as a precursor for the synthesis of porous carbons by hydrothermal carbonization (HTC) and pyrolysis. Waste marigold flowers were hydrothermally treated at 180 °C for 2, 12, and 24 h. The resultant hydrochars were subsequently pyrolyzed at 800 °C under argon (Ar) atmosphere. Based on X-ray diffraction and Raman spectroscopy analyses, the samples exhibited an amorphous phase regardless of HTC time. With increasing HTC time, the marigold surface became rougher and more ruptured. This resulted in the development of a porous structure, thereby increasing surface area. The specific surface area of carbon samples increased from 118 to 281 m2 /g with HTC increasing from 2 to 24 h, respectively. Increase of specific surface area mainly resulted from the development of a microporous structure at longer HTC times. Our results offer guidelines to control surface area and porosity through the adjustment of HTC conditions.