A study of complex organic and warm carbon-chain molecules in the earliest Class 0/I stage of star-formation is essential to probe the initial conditions of star and planet formation. These organic molecules have been observed toward high- and low-mass protostars yet have never been detected among the Class 0/I proto-brown dwarfs (proto-BDs). We have conducted the first deep molecular line survey of organic chemistry in proto-BDs. Several transition lines of the fundamental complex organic molecule, methanol, are detected at upper state energy levels of ~7-49 K probing critical densities of 10^5 to 10^9 cm^-3. The median methanol column density for the proto-BDs is a factor of ∼3 higher than the median for low-mass protostars. In addition, various carbon-chain molecules have been detected for the first time in a proto-BD, with abundances comparable to low-mass protostars. The detection in the high-excitation lines of these molecules provides strong evidence of the presence of a warm corino in proto-BDs, as predicted by brown dwarf formation models.