This paper investigates the enhancement of fly ash-based geopolymer with chitosan biopolymer. Unconfined compression and split tensile tests were carried out to investigate the effect of addition of small amount of N-carboxymethyl chitosan (0.05, 0.1, 0.15, and 0.2 wt% of fly ash) on the mechanical performance of fly ash-based geopolymer. Scanning electron microscopy (SEM) imaging was also conducted to study the microstructure of the chitosan enhanced fly ash-based geopolymer. The results indicated that the inclusion of N-carboxymethyl chitosan led to slight increase of the unconfined compressive strength and substantial increase of the tensile strength, the displacement at the peak tensile load and the pre-peak toughness, with the maximum increases at 0.1 wt% chitosan content. The SEM imaging indicated that the added N-carboxymethyl chitosan biopolymer coated and bridged the (geopolymerized) fly ash particles and led to the formation of a more condensed geopolymer network structure, thus enhancing the mechanical behavior of the geopolymer–biopolymer composite. However, when too much N-carboxymethyl chitosan was used, the excessive coating and encapsulation of un-reacted and partially hydrolyzed fly ash particles hindered their geopolymerization and adversely affected the mechanical behavior of the geopolymer–biopolymer composite.