Mass timber is emerging as a viable form of construction around the world in new markets for wood buildings. The entrance into these markets has driven the demand for more knowledge to enable designs alongside other structural materials such as steel and reinforced concrete. Large, in-plane tests on cross-laminated timber (CLT) diaphragms (4570 mm x 4570 mm [15 ft x 15 ft]) are used to quantify ductility through the diaphragm force reduction factor (Rs) from ASCE 7-16, m-factors from ASCE 41-17, and validate common methodologies of mass timber design currently implemented in structural engineering practice. The tests demonstrate that cross-laminated timber can function well as a diaphragm with a mean Rs value of 1.19 at indicate a ductility like precast concrete diaphragms with R_s=0.7-1.4. Like precast concrete systems, cross-laminated timber diaphragms depend heavily on the inter-panel connections for a ductile design and will require several categories to classify the types of CLT systems. Analysis methods from ASTM E455 validate the assumptions that a CLT diaphragm is shear-controlled in its behavior for purposes of determining Rs. M-factors are an indirect measurement of the nonlinear deformation capacity of a component and are used as a multiplier to the expected strength of a component. The m-factors observed (0.46 to 1.9 for Immediate Occupancy to Collapse Protection performance levels, respectively) resulted in lower than values from previous studies on similar panel-to-panel connections. The initial stiffness of the large diaphragm panel-to-panel connections, 6.86 kN/mm (39.8 kip/in), were lower than the spline stiffness estimates of 11.5 kN/mm (65.7 kip/in) based on individual fastener tests. The hysteretic loading resulted in lower spline stiffnesses 4.37 kN/mm (24.9 kip/in) while the monotonic testing showed a mean spline stiffness of 9.04 kN/mm (52.5 kip/in). Calculating CLT diaphragm displacement based on NDS methods proved to be conservative compared to test results for the purposes of determining ASCE 7-16 diaphragm flexibility status.