Mack was born in Schriesheim, Palatinate in contemporary Baden-Württemberg, Germany, where he worked as a miller. He was born the third son to miller Johann Phillip Mack and his wife Christina Fillbrun Mack and baptized into the localReformed church on 27 July 1679. The Macks remained in Schriesheim throughout the Nine Years' War, intermittently seeking refuge in the hill country because of violence. Upon finishing his studies, Mack took over the family mill and married Anna Margarethe Kling on 18 January 1701. By 1705, the Macks became moved by the Pietist movement locally led by Ernst Christoph Hochmann von Hochnau and started to host an illegal Bible study and prayer group at their home. In the early 1700s, Graf Henrich AlbrechtSayn-Wittgenstein provided refuge to religious dissenters from other German states and elsewhere. Many were settled around the small village of Schwarzenau, including Mack and his followers. The era of toleration for radical Pietism lasted only until ~1740, but had few precedents at the time and was denounced by the rulers of most other German states. Schwarzenau is now part of the town of Bad Berleburg in the district of Siegen-Wittgenstein in the state of Nord Rhein Westfalen. The school in Schwarzenau was named in honor of Alexander Mack. The initial group that became known as the Schwarzenau Brethren were inaugurated by Mack as a Bible study with four other men and three women. In 1708—having become convinced of the necessity of Believer's baptism—the group decided to baptize themselves, using a lottery system to choose who would baptize one another in the Eder.
In 1719, a branch of the Schwarzenau Brethren led by Peter Becker emigrated to Germantown, Philadelphia, Pennsylvania, United States for religious freedom. Mack and several other Brethren emigrated to Friesland in the Netherlands because of pressure within the interfaith community in Schwarzenau in 1720. However, the Brethren in Friesland were impoverished, and the community was unable to sustain itself. In 1729, Mack and about 30 Brethren families sailed for Germantown. The arrival of the 1729 emigrants immediately brought renewed vitality and new members to the American Brethren, who had become less active since their migration. Mack's leadership was pivotal in sustaining this vitality, and it noticeably slackened upon his death in 1735.
