We report on a pump-probe thermal lensing method for measuring the linear absorption coefficient of liquids by using interferometry and numerical analysis. The method is based on interferograms generated when a localized photothermal effect is induced in the sample. The photothermal effect itself is induced by a pump beam impinging on a sample located on-axis of the probe beam, which is one of the paths of a Mach-Zehnder interferometer. A digital camera is employed as the acquisition device allowing the capture and storage of the experimental data. During the experiment, a total of three photographs are taken and stored on a personal computer, and by using an algorithm, the numerical analysis is done. Numerical analysis is subsequently used to calculate the photothermal phase difference and the normalized spatial distribution of the pump beam irradiance. Plotting the phase difference as a function of the spatial distribution of the pump beam produces a linear dependence from which the linear absorption coefficient is obtained. The sensitivity of the method (λ/1500) is validated using ethanol, methanol, and carbon disulfide.