Background: To approach the mammographic X-ray investigations in a metrological manner, any potential improvement needs to be considered. In this study, the influence of skin thickness variation on the mammographic X-ray spectra was investigated. Additionally, some attenuation parameters such as linear and mass attenuation coefficients, mean free path, and half-value layer thickness were assessed for exposure of the skin to the mammographic X-rays. Materials and Methods: A theoretical experiment was performed using MCNP6.2 by simulation of two collimators, a detection area, a skin sample, and a point monoenergetic X-ray source. Carrying out this experiment, the above-mentioned parameters were estimated. As a validation process, the mass attenuation coefficients obtained by MCNP were compared with the results of XCOM. Results: The good agreement between the results of MCNP and XCOM reveals that the geometrical setup was modeled correctly by MCNP. It also implies the reliability of the Monte Carlo technique for attenuation calculations of low-energy X-rays through the skin. Upon increasing the breast skin thickness, its transmission factor against mammographic X-rays decreases linearly, and it increases by increasing the photon energy. The older the patient, the more transmission factor of the breast skin against the low energy X-ray. Conclusion: Increasing the X-ray energy leads to less sensitivity of the transmission factor to the skin thickness. Thus, to have the best possible quality of X-ray mammography images, the patient’s age may also be considered as well as the other factors, when selecting the proper energies and for fine-tuning the used X-ray generators.