The Triennial Convention, formally the General Missionary Convention of the Baptist Denomination in the United States of America for Foreign Missions, was formerly the name for the mainline Baptist denomination in the United States. The Triennial Convention was established in 1814 in Philadelphia, Pennsylvania (its headquarters), mainly to advance missionary work. The Convention assembled every three years, and because of that was named as "Triennial" Convention. In a controversy over slavery and missions policy, most churches in the South split from the Triennial Convention and founded the Southern Baptist Convention in May 1845, before the Civil War. This event left the Convention largely northern in its composition of churches and membership. In 1907, the Triennial Convention was reorganized as the Northern Baptist Convention, now known as the American Baptist Churches USA.

Distinguished from other Christian and Protestant traditions by their commitment to credobaptism and high local church autonomy, the Baptist tradition have been present in the United States since Roger Williams founded the First Baptist Church in America in Providence, Rhode Island in 1638. Baptist churches were soon found elsewhere in colonial America. The First Baptist Church of Boston was founded in 1665, and Pennepack Baptist Church in Philadelphia, Pennsylvania, was organized in 1688. The founding of First Baptist Church of Charleston, South Carolina, in the late 1690s marked the spread of Baptists to the South.

In the 18th and 19th centuries, Baptists began forming regional associations and societies to foster cooperation in missionary, benevolent, and educational work, such as the founding of Brown University. Associations could determine their own standards for fellowship and offer advice to churches, but local congregations governed themselves and ordained their own ministers. The first permanent Baptist association in America was the Philadelphia Association, established in 1707.

The Second Great Awakening inspired the establishment of foreign missions agencies to spread the Christian religion throughout the world. In 1810, the Congregationalists established the American Board of Commissioners for Foreign Missions. Two years later, the Congregationalist Board sent Adoniram Judson, Jr. (1788–1850), Ann Hasseltine Judson (1789–1826), and Luther Rice to India. Upon arrival, however, the three missionaries repudiated infant baptism and became Baptists under the influence of British missionary William Carey (1761–1834), a founder of Britain's Baptist Missionary Society.

Carey and the three American missionaries mobilized Baptists in America to support the Judsons' planned mission trip to Burma. Their efforts led to the establishment in 1814 of the General Missionary Convention of the Baptist Denomination in the United States of America for Foreign Missions. The Convention was tasked with collecting funds from Baptist groups and individuals to support foreign missions. The Convention was called "Triennial" because the national body assembled every three years. Members of the denomination were called American Baptists or Triennial Baptists. At the Triennial Convention's first assembly between May 18 and 25, 1814, Richard Furman was elected president, the Baptist Board for Foreign Missions was created, and the denomination sent missionaries to China, Africa, and South America. Other state conventions, regional associations, and societies were being established, such as the Baptist General Tract Society (later renamed American Baptist Publication Society) in 1824 and the Home Mission Society in 1832. The various societies held their own conventions during sessions of the Triennial Convention.

By 1840, Baptists were in every state and territory as well as missions around the world. Alongside the Methodists, Baptists had grown to be one of the two largest denominations in the United States. Nevertheless, there were Baptists who opposed efforts to establish missions boards and denominational agencies as unbiblical. These Baptists became known as "anti-mission" or Primitive Baptists, while those who supported organized missionary work became known as Missionary Baptists. As early as 1838, African-American Baptists began organizing their own independent associations and conventions. Immigrants, such as Danish, Norwegian, Swedish and German Americans, also formed their own Baptist denominations along ethnic lines rather than affiliate with the Anglo-American oriented Triennial Convention.

The Triennial Convention attempted to take no stated position on slavery. This moderate position allowed both abolitionists and slavery supporters to remain in the denomination. The majority of Triennial Baptists in the Northeast opposed slavery, while the growing number of Triennial Baptists in the Southeast supported slavery.^{[citation needed]}

In 1843, the abolitionists in the Northeast founded the Northern Baptist Mission Society in opposition to slavery. In 1844, the Home Mission Society refused to ordain James E. Reeve of Georgia as a missionary because he was put forward as a slaveholder. They refused to decide on the basis of slavery. In May 1845, in Augusta, Georgia, the slavery supporters in the Southeast split from the Triennial Convention and founded their own denomination: the Southern Baptist Convention (SBC). The Triennial Convention became largerly northern, with the abolitionists, after the Southern schism. The Northern Baptist Mission Society was dissolved. After the schism, authorization was given to rename the Foreign Missions Board as American Baptist Missionary Union. William Bullein Johnson joined the Southern Baptist Convention.