Mizner served as President of the California Senate and was US Minister (ambassador) to Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua.

His stepfather, James Semple, was fluent in the Spanish language and was appointed Chargé d'affaires to the Republic of New Granada (present-day Colombia and Panama, and pieces of neighboring countries).

He arrived in California by way of New Orleans and Panama May 20, 1843 (not 1849), settled at Benicia, and became a partner in the general merchandise company of Semple, Robinson & Co.

At the age of 22 he joined the United States Army, and served in the Mexican War on the staff of General Shields.

During the administration of President Benjamin Harrison, Mizner was appointed in 1889 Minister to Central America, in essence the ambassador to Costa Rica, El Salvador, Guatemala (where he was based), Honduras, and Nicaragua (Panama at that time was part of Colombia).

There was a complicated incident in which American arms were being shipped to El Salvador (which had just had a coup d'état), and Guatemala, having martial law, objected.

He had seven children, one daughter Minnie, married to Horace Blanchard Chase, who later would found Stag's Leap Winery, and six boys: William, Lansing, Edgar, the architect Addison, Harvey, and Wilson.