Engelhardt, known as the "Father of Mission History,"[by whom?] compiled extensive histories of the twenty-one Spanish missions in Alta California as well as other Franciscan settlements in Baja California and Arizona during the first decades of the 20th century. Engelhardt's work is today considered to be the standard authority regarding California mission history.
His many works include:
The Franciscans in California (1897)
The Franciscans in Arizona (1899)
The Missions and Missionaries of California (1908–1915)
Engelhardt also worked with the Pomo Native Americans of Northern California and compiled the vocabulary of their language.