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Kīlauea Volcano on Hawai'i Island is host to a complex volcanic and interwoven fault system. Over the last ∼120 years, a range of seismic events, including large earthquakes such as the 1975 Mw ${M}_{w}$7.7 Kalapana earthquake, creep, and slow slip events, have occurred along the décollement underlying Kilauea's south flank. We explore both the deformation and stress changes of Kīlauea from 1896 to 2018 by collating six geodetic data sets and creating an analytical model to determine the dominant deformation sources (i.e., fault planes, rifts, magma chambers) driving this system at different times. The 1975 Kalapana earthquake significantly altered the region's state of stress and deformation; we find the average slip along the décollement was reduced from 10 cm/yr prior, to 4 cm/yr after the rupture. Prior to 1975 no slip is resolved along the décollement where the earthquake nucleated, suggesting that this portion may have been locked leading up to the rupture. After 1975, décollement slip overall is smaller and more irregular, suggesting increased control by spatial variation of mechanical properties. We find increases in shear stress along the Kīlauea décollement and a decrease in normal compressive stress within the East Rift Zone prior to the Kalapana earthquake, creating favorable conditions for failure of the décollement and subsequent magmatic intrusion. Plain Language Summary: Kīlauea Volcano on Hawai'i Island encompasses a complex volcanic and interwoven fault system. The low‐angle fault underlying the volcano has consistently been moving it southward, and has a high hazard potential due to large magnitude earthquakes. The complexity of this region leads to questions about the evolution of deformation and its seismic cycle. The 1975 Mw ${M}_{w}$7.7 Kalapana décollement earthquake is of particular interest because large magnitude ruptures can dramatically alter the state of stress within a region. We therefore explore both the deformation and stress changes of Kīlauea's décollement from 1898 to 2018 by collating a wealth of surface displacement observations. We create a model to reproduce the observed displacements throughout time and to identify key structural features causing the deformation, such as fault planes, rifts, and magma chambers. Multiple deformation sources were needed to model the observations, with more of these sources required prior to the 1975 Kalapana earthquake than afterward. We also find varied patterns and magnitudes of deformation and stress both temporally and spatially within the region. Kīlauea's history has important implications for our understanding of the relationship between magmatic and earthquake cycle processes. Key Points: Collation of ∼120 years of geodetic data at Kīlauea reveals key displacement, stress, and deformation source patterns throughout timeGreater and more complex displacement occurred within Kīlauea's south flank prior to the 1975 MW ${M}_{W}$7.7 Kalapana earthquake than after itCompressive stress decreased in East Rift Zone before/after 1975; shear stress increased before and decreased after on Kīlauea décollement [ABSTRACT FROM AUTHOR]