Micro-electro-mechanical-systems (MEMS) prefer electrostatics as their actuation technique. In the literature, MEMS have been generally analysed as a combination of a Bernoulli-Euler microbeam-type deformable electrode, stationary ground electrode and an electrostatic potential difference applied between them. There exists a microbeam configuration-dependent inherent upper threshold on the amount of voltage that can be applied between electrodes. If the applied voltage is increased beyond this value, the deformable electrode shifts from a stable to an unstable configuration, thereby snapping on the ground electrode. This critical value of the applied voltage is termed as the pull-in voltage and the corresponding maximum microbeam displacement is termed as the pull-in displacement. The beam lateral shear starts to play significant role as the beam thickness-to-length ratio increases. In this paper, details with regard to the Timoshenko beam theory-based spectral finite element (TBT-SFE) for the determination of static pull-in instability parameters of narrow shear deformable microbeams, thereby taking non-linearity associated with forcing terms as well as beam lateral shear of the first-order into account, are presented. The authors have investigated an appropriate combination of number of nodes per element and total number of elements of the proposed TBT-SFE to carry out the aforementioned study. The authors have also presented effects of the electrostatic fringing field on static pull-in instability parameters of microbeams having different values of the beam thickness-to-length ratio. This paper highlights the importance of the electrostatic fringing field and beam lateral shear on static pull-in instability parameters of narrow shear deformable microbeams.