From a climate dynamics perspective, air mass persistence reflects variability in the dynamic nature of the atmosphere. In this study, a historical analysis of synoptic air mass persistence across the continental United States is presented to portray spatial and temporal variability and trends in air mass residence times. Historical daily air mass calendars for 140 locations across the United States for the 60-year period 1955 through 2015 were extracted from the Spatial Synoptic Classification database. The data were stratified by season, and a historical climatology of seasonal air mass occurrence was created for each location. The historical daily air mass data were then translated into a record of residence time, or the length of consecutive days that a synoptic air mass type was in place at a location. Each historical record of seasonal air mass residence times, or persistence, was then analyzed for spatial variability across the United States and for temporal variability and trends. Results reveal a statistically significant increase in air mass persistence for many areas of the country during three seasons, but most commonly across the southern United States during the summer season (June-August). However, this pattern was reversed for the winter season (December-February), the analyses revealed a general pattern of decreasing cool-season air mass persistence across the continental United States. The seasonally-dependent change in air mass persistence across the United States may be indicative of changed or changing mid-latitude atmospheric dynamics in the form of a previously suggested northward migration of the polar jet stream.