[1] On 28 September 1864, Eccarius was a participant at the first meeting of the International Workingmen's Association (IWA) at St. Martin's Hall in London.
[2] Eccarius would there be chosen as a member of the governing General Council, a position he would retain until the transfer of the headquarters of the International to America in 1872.
[2] Eccarius would be a fixture of the IWA, attending every international conference and congress of that organization for its entire 8-year duration in Europe.
[3] Upon the nomination of journalist Peter Fox, Eccarius was unanimously elected the new head of that organization.
[2] From 1870 until 1872, Eccarius was the Corresponding Secretary of the IWA for America, serving as the point of contact with the organization's adherents on the other side of the Atlantic.