Several locations across the United States in non-compliance with the national standard for ground-level ozone (O₃) are thought to have sizeable influences from distant extra-regional emission sources or natural stratospheric O₃, which complicates design of local emission control measures. To quantify the amount of long-range transported O₃ (LRT O₃), its origin, and change over time, we conduct and analyze detailed sensitivity calculations characterizing the response of O₃ to emissions from different source regions across the Northern Hemisphere in conjunction with multi-decadal simulations of tropospheric O₃ distributions and changes. Model calculations show that the amount of O₃ at any location attributable to sources outside North America varies both spatially and seasonally. On a seasonal-mean basis, during 1990-2010, LRT O₃ attributable to international sources steadily increased by 0.06-0.2 ppb yr^-1 at locations across the United States and arose from superposition of unequal and contrasting trends in individual source-region contributions, which help inform attribution of the trend evident in O₃ measurements. Contributions of emissions from Europe steadily declined through 2010, while those from Asian emissions increased and remained dominant. Steadily rising NO_x emissions from international shipping resulted in increasing contributions to LRT O₃, comparable to those from Asian emissions in recent years. Central American emissions contribute a significant fraction of LRT O₃ in southwestern United States. In addition to the LRT O₃ attributable to emissions outside of North America, background O₃ across the continental United States is comprised of a sizeable and spatially variable fraction that is of stratospheric origin (29-78%).