William Horace Warner (8 May 1812 – 26 September 1849) was an officer in the United States Army's Corps of Topographical Engineers. In 1849, he led an Army survey party north from Sacramento through the uncharted country of northeastern California into south central Oregon. Warner was killed by Native Americans in northeastern California, just south of the Oregon border. In the mid-nineteenth century, two army outposts in southern Oregon were named after Warner. Today, the Warner Mountains, Warner Valley, and a number of other landmarks bear his name.

Warner was born on 8 May 1812 in Columbia County, New York. He attended West Point, entering the school in July 1831 and graduating on 1 July 1836. Warner graduated from West Point, tenth in a class of 49 cadets. Upon graduation, he was commissioned as a second lieutenant and assigned to the United States Army's 1st Regiment of Artillery.

As a second lieutenant, Warner served in the Second Seminole War from mid-1836 until 1838. He was then assigned to help oversee the movement of the Cherokee Nation to the west. On 7 July 1838, Warner was appointed to the Corps of Topographic Engineers, retaining his rank of second lieutenant. As a topographic engineer officer, Warner was placed in charge of a project to improve the harbor at New Bedford, Massachusetts. He led that effort until 1839, when he was directed to survey of Sackets Harbor in New York. Later that year, Warner was sent back to Florida to support action in the ongoing war with the Seminole people. Then in 1840, Warner was directed to survey the Detroit River in Michigan. He finished that assignment in 1841.

On 1 September 1841, he was promoted to first lieutenant in the Corps of Topographical Engineers. After his promotion, Warner was sent to do a reconnaissance of the approaches to New Orleans in Louisiana. After completing that job in 1842, he was assigned to conduct a general survey of Lake Michigan and make recommendations for improving harbors around the lake. That effort lasted into 1843. Warner was then directed to conduct a survey of the Ohio River near Pittsburgh, Pennsylvania. After finishing that project in 1845, Warner was sent to survey the boundary between the United States and the British provinces of Canada.

Warner was actively engaged in the Mexican–American War. He was assigned to Brigadier General Stephen W. Kearny's California expedition. On 6 December 1846, Warner participated in the Battle of San Pasqual, where he was wounded. After the battle, he was recognized for gallantry in the field and promoted to Brevet Captain.

From 1847 to 1849, Warner was assigned to do survey work in California. In May 1847, he surveyed Alcatraz Island. His work was the first detailed examination of the island. In May 1848, Warner began a survey of the military reservation at the Presidio in San Francisco. Later in 1848, Warner surveyed the Sacramento town site with the assistance of Lieutenant William T. Sherman.

In October 1848, Warner joined Lieutenant Sherman, Colonel Richard B. Mason (the military governor of California), and a civilian named Norman S. Bestor to form a mercantile company. Each of the four investors contributed $500 to the venture. Their plan was to open a general store in Coloma, California to sell supplies to miners. After the company was formed, the business was turned over to Bester to run with the help of a clerk who had previously worked for Warner.

In January 1849, Warner participated in the first criminal trial held in Sacramento County, California under United States law. The trial prosecuted a store keeper named Charles F. Pickett for the murder of a rival merchant. Warner attended the public meeting that appointed a judge and prosecutor for the trial. A few days later when the trial occurred, Warner was selected to serve on the jury. The trial ended in a deadlocked jury and Pickett was freed on bond.