Mechanical properties of organic molecular semiconductors are determined by a combination of chemical structure and solid-state packing. Measurements of nanoscale mechanical properties on molecular surfaces via atomic force microscopy (AFM) are particularly challenging as the very act of probing how stiff these surfaces are may perturb them, making it difficult to discern subtle differences in stiffness arising from changes in molecular composition. This work presents the first direct, experimental demonstration of the tunability in the nanomechanical properties for a family of molecular semiconductors resulting from systematic alkyl sidechain substitution. While such tunability is intuitively expected, it is a subtle effect that is extremely difficult to detect and quantify reliably from nanoscale AFM measurements due to various spurious force contributions operating on such small length scales. Only after identifying and removing these spurious contributions is the underlying molecular-scale tailoring of mechanical properties observable. Confidence in the measured stiffness trend is reinforced through simulations based on density-functional theory (DFT) and molecular dynamics (MD).