Mercy High School, originally named Our Lady of Mercy Academy, was established by the Sisters of Mercy in 1882. During its early years, Mercy Academy educated children in grades K-12. A boarding facility was also available for high school age students. The original school building was located two blocks north of the present high school. In 1959 the elementary and high school separated and Mercy High School moved to its current site.

The Sisters of Mercy and other clergy taught most classes until 1960. Their goal of educating young people in a strong academic and religious environment fostered growth in the school, and as enrollment grew, lay teachers were added to the faculty. By 1983, only the principal and vice principal were Sisters of Mercy. In 1984, Mercy High School became a diocesan school within the Roman Catholic Diocese of Sacramento. Mercy High is still a reflection of the ideals the Sisters of Mercy brought to Tehama County over 100 years ago.

The school offers a rigorous college preparatory program in a supportive, close community setting. Advanced Placement and Honors programs are available for qualified and interested students. The Mercy athletic and fine arts departments encourage all students to compete in sports activities and school productions. This diverse educational setting encourages all students to reach their highest potential and is the trademark of a Mercy education.