South Lancaster Academy (PreK-12) opened its doors as an academy on April 19, 1882. Advanced work was carried for several years in theology, teacher training, and business before its standing as a junior college was formally recognized in 1918. At that time the name Lancaster Junior College was adopted. By 1922, a four-year theological course was being offered and degree granting powers were conferred by the Massachusetts legislature. At the same time, the name was changed to Atlantic Union College. In 1922, the academy and college became separate institutions with the academy having its own board of trustees and faculty.
Since 1967, SLA has been a twelve-grade school. SLA maintain the historic ideals of Seventh-day Adventists (SDA) on matters of morals, dress, and conduct as its reasons for existence. SLA is operated by the SDA churches of Atlantic Union College, Leominster, South Lancaster Village, Lancaster Portuguese and Sterling. South Lancaster Academy is affiliated with the Southern New England Conference of Seventh-day Adventists and the Atlantic Union Conference of Seventh-day Adventists.