The Mennonite religion currently has about 1.5 million followers (2006). This anabaptist religion is named after Frisian Menno Simons, who was responsible for formalizing the teachings of other earlier Swiss founders. The Mennonites emigrated from Switzerland in several distinct groups, including those who went to Russia before coming to America and those who traveled down the Rhine to Holland, England, and then the United States.
The Mennonites are deeply involved in keeping records of their ancestors and descendants. An extensive collection of genealogical materials is maintained by the Lancaster Mennonite Historical Society at its facility in Lancaster, Pennsylvania. They also issue a monthly publication, Pennsylvania Mennonite Heritage, which contains extensive articles about the early Mennonite ancestors and their religion.The LDS Family History Library maintains a complete collection as well as the indexes. The indexes also available as online PDF downloads.
The Mennonite ancestry is Celt, but these earlier peoples did not maintain a written language. The only records of them are those pieced together from the writings of their Roman conqueror, Julius Caesar and archeological findings. The earliest remains of any Celtic people are of those trapped in a salt mine near Zurich, Switzerland.
The FHL maintains copies of the family histories of many early Mennonite immigrant ancestors, as well as Milton Snavely Hershey, the founder of the confection and food company that bears his name.
In addition to many records from Lancaster County, Pennsylvania, which continues to have one of the largest populations of Mennonites and related religions (Amish, Dunkards, Church of the Brethren) in the United States, there are a number of sites of particular interest to genealogists tracing 19th and 20th century Mennonite families.
Mennonite Church USA
Churches -- For locating Mennonite Congregations by name or location, is Mennonite.net (note that not all Mennonite congregations are associated with this site).
Mennonite Archives -- Online scanned archives images and documents of genealogical interest are in the Gutenberg to Gigabytes: Archives for the 21st Century collection.
Obituaries -- The on-going collection of Mennonite Obituaries