Mill and thin asphalt overlays are a popular preservation treatment for asphalt and composite pavements owing to their ability to address a variety of distresses. Their effectiveness depends on different factors such as traffic, climate, and condition of the pavement at the time of treatment, with typical treatment lives ranging from 5 to 12 years. In cold regions, performance in relation to roughness is of particular interest, as factors like frost action and thermal cracking tend to accelerate roughness progression. The objective of this study was to evaluate the effect of thin asphalt overlays on pavement roughness in cold climate locations using a set of field performance data from the Pavement Preservation Group Study, a broader research study conducted by the National Center for Asphalt Technology and the Minnesota Department of Transportation’s Road Research Facility. The findings indicated that treatment application resulted in both short- and long-term benefits. Following construction, a performance jump was obtained, which was greater for pavement sections with higher International Roughness Index (IRI) IRIpre values. This can be interpreted as rougher sections benefiting more in the short term than smoother sections, where the IRI is already low. Conversely, pavements with lower roughness at the time of treatment obtained greater life-extending benefits. This is interpreted as smoother pavements obtaining a greater benefit over time, and reinforces the concept of preserving pavements while in good condition to maximize pavement life. The magnitude of the benefits was also dependent on the type of milling machine used.