[1] The city is the largest community in the region of far eastern Oregon, also known as the Western Treasure Valley.

Ontario was founded on 11 June 1883, by developers William Morfitt, Mary Richardson, Daniel Smith, and James Virtue.

Soon after, stock began arriving from Eastern Oregon's cattle ranches to Ontario's stockyard for transshipment to markets throughout the Pacific Northwest.

In addition, the construction of the Nevada Ditch and other canals aided the burgeoning agricultural industry, adding those products to Ontario's exports.

Smith told the Associated Press "If the Japs, both alien and nationals, are a menace to the Pacific Coast safety unless they are moved inland, it appears downright cowardly to take any other stand than to put out the call, 'Send them along; we'll cooperate to the fullest possible extent in taking care of them.'"

According to the United States Census Bureau, the city has a total area of 5.17 square miles (13.39 km2), all land.

[9] As is common with the high deserts of Oregon, winters are cold and snowy, while summers are hot, dry and sunny.

As of the 2020 United States census, there were 11,645 people, 4,315 households, and 2,491 families residing in the city.

The Heinz Frozen Food Company (formerly Ore-Ida), a subsidiary of H. J. Heinz Company, processes locally grown potatoes, and annually produces over 600,000,000 pounds (270,000,000 kg) of 75 different potato products, while employing approximately 1,000.

[19] St. Alphonsus Medical Center is a 49-bed, acute-care hospital, serving Ontario and the surrounding communities in Eastern Oregon and southwestern Idaho; it is part of the hospital system of Saint Alphonsus Regional Medical Center in Boise, Idaho.

[20] About five miles (8 km) northwest of central Ontario is the Snake River Correctional Institution, a 3,000-bed medium security facility.

Since 2019, cannabis dispensaries have been an important part of Ontario's economy and a notable example of the border effect.

As Ontario borders Idaho (where marijuana remains illegal for all purposes), local news outlets reported that by November 2019 the city's dispensaries were generating a significant amount of revenue from Idaho residents driving across the state line.

[22] Total cannabis sales from Ontario dispensaries topped $100 million in December 2020.

[23] The additional revenue from cannabis sales reversed a years-long trend of cutting the annual city budget and stoked advocacy for creation of a Greater Idaho, as many in that part of the state did not vote to legalize recreational marijuana or for the decriminalization of harder drugs along with the more liberal parts of the state, as well as those in Idaho who feel the border purchases of marijuana have increased drug abuse and hurt their way of life there.