Starting at the Prime Meridian and heading eastwards, the parallel 42° north passes through:
|Country, territory or sea||Notes|
|Island of Corsica|
|Mediterranean Sea||Tyrrhenian Sea|
|Sea of Japan|
|Island of Hokkaidō|
| Oregon / California border|
Oregon / Nevada border
Idaho / Nevada border
Idaho / Utah border
| Pennsylvania border|
New York / Pennsylvania border
The Adams-Onís Treaty of 1819 established the parallel 42° north as the border between the Viceroyality of New Spain of the Kingdom of Spain and the western territory of the United States of America from the meridian of the headwaters of the Arkansas River west to the Pacific Ocean. The Treaty of Guadalupe Hidalgo of 1848 then ceded much of what was then northern Mexico to the United States; as a result, the northernmost U.S. states which were created from Mexican territory (California, Nevada, and Utah) have the parallel 42° north as their northern border, and the adjoining U.S. states of Oregon and Idaho have the parallel 42° north as their southern border.
The parallel 42° north passes through the southern end of Lake Michigan and Lake Erie. Part of the water boundary between the United States and Canada passes south of the 42nd parallel, and the southern tip of the Canadian province of Ontario just barely goes south of it - this is located at Point Pelee and Pelee Island.
The parallel passes through the states of Wyoming, Nebraska, Iowa, Illinois, Michigan, Pennsylvania, New York, Connecticut, Rhode Island and Massachusetts, and passes through (or near - within one-half degree of latitude) the following cities in the United States: Medford, Oregon, Yreka, California, Des Moines, Iowa, Chicago, Illinois, Detroit, Michigan, Cleveland, Ohio, Erie, Pennsylvania, Binghamton, New York, Hartford, Connecticut, Providence, Rhode Island, and Boston, Massachusetts.