Ontario, California Wikipedia open wikipedia design. Ontario is a city of California in the United States. It is located 35 miles east of downtown Los Angeles. This short article about a place or feature in the United States can be made longer. You can help Wikipedia by adding to it. This page is based on a Wikipedia article written by contributors (read/edit).Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.