Hybrid structural systems, consisting of concrete or steel vertical Lateral Load Resisting Systems (LLRSs) and mass timber (MT) horizontal diaphragms, are one of the best structural solutions for sustainable tall buildings. The research on the in-plane performance and modeling of MT diaphragms in tall hybrid buildings has been relatively limited so far. This project was funded by Forestry Innovation Investment (FII) to conduct research with a focus on quantifying the in-plane performance of MT diaphragms in tall hybrid buildings subjected to lateral (seismic and wind) loads to support the use of MT diaphragms in hybrid wood/non-wood systems. In this study, the overall diaphragm design methodology and main aspect related to the (capacity and deflection) design of the drags, collectors, splines, and other parts of the diaphragm were defined. Analysis and modelling approaches for MT diaphragms were summarized. Specific derivation methods for modelling input, e.g., stiffness properties of MT panels and connections, were also developed. The interaction between the MT diaphragm and the vertical LLRS during a seismic response was discussed. In collaboration with Glotman Simpson Consulting Engineers, a 17-storey timber-concrete hybrid building was designed and analysed as a case study in this project. This study will help mass timber penetrate taller hybrid buildings that are either not currently permitted or not used to their potential due to the lack of design details and guidelines. The developed information can be shared with other firms with similar expertise.