In-situ catalytic fast pyrolysis (CFP) is considered to be a promising pathway to produce aromatic hydrocarbons from lignocellulosic biomass. However, the presence of variable amounts of inorganic ash in biomass in the form of alkali and alkaline earth metals (AAEMs) is a concern while using in situ catalysts because AAEMs could influence product distribution from CFP while also being a major reason for catalyst deactivation. In this study, the effect of four alkali and alkaline earth metals (K, Na, Mg, and Ca) commonly found in biomass was investigated to understand their individual influence on the fate of primary pyrolysis products as well as their effect on the selectivity of products from in situ CFP using ZSM-5 catalyst. Experiments were performed in a microreactor (Py-GC/MS) with ZSM-5 catalyst using AAEM-impregnated biomass. It was found that the type of AAEM as well as the concentration were significant, with Mg appearing to be relatively inert when compared to the stronger catalytic activity of K, Na, and Ca. The influence of AAEMs on the formation of pyrolysis products from cellulose, hemicellulose, lignin, and its subsequent influence on CFP is discussed. From noncatalytic pyrolysis experiments, even the lowest concentration of AAEMs (0.1 wt %) was observed to have a significant influence on the thermal decomposition behavior of biomass, promoting the formation of lower molecular weight cellulose and lignin-derived products. AAEMs were found to be influencing CFP product distribution by reducing the carbon yield of desired aromatic hydrocarbons and olefins, while it accelerated pathways resulting in increased yields of thermally derived char and noncondensable gases. The effect of AAEMs on CFP followed the order: Na > K > Ca > Mg.