In 1866, Union veterans of the Civil War organized into the Grand Army of the Republic (GAR). Membership in this fraternal organization was limited to veterans of the Union Army, United States Navy, United States Marines and United States Revenue Cutter Service who served in the American Civil War. Organized into departments at the state level and posts at the community level, the GAR became the preeminent veterans' organization. Massachusetts had 210 local GAR posts. Its members actively participated in political discourse, supporting voting rights for black veterans, lobbying the United States Congress to establish veterans' pensions, and supporting Republican political candidates. Its peak membership was in 1890 with more than 400,000 members nationally.

The Philip H. Sheridan, Post #34 of Salem, Massachusetts was organized on December 10, 1867. Charter members included Captain George M. Whipple of the 23rd Regiment Massachusetts Volunteer Infantry. This post quickly became the largest in the state with nearly 600 names on the muster roll. The General Lander, Post #5 of Lynn, Massachusetts was organized in February 1867. In 1885, members of this post built the Grand Army of the Republic Hall located on 58 Andrew Street in Lynn, Massachusetts as a memorial to the Union Army Veterans of the Civil War. The Hall is still located in Lynn and was added to the National Register of Historic Places on May 7, 1979. The John H. Chipman, Jr., Post #89 of Beverly, Massachusetts was organized on June 5, 1869. In 1882, members of this post helped raise the funds necessary to erect the soldiers' monument at the junction of Abbott and Endicott streets. The A. W. Bartlett, Post #49, of Newburyport, Massachusetts was organized on February 20, 1868. In the 1900s, members were active in social affairs and community interactions, often in conjunction with female auxiliaries and other local groups. They participated in parades, patriotic celebrations, grave decoration, social outings, memorial services, and communicated with other Massachusetts veteran organizations.

The national office of the GAR closed in 1956. The GAR was succeeded by the Sons of Union Veterans of the Civil War (SUVCW), composed of male descendants of Union veterans.