Steady, cold rain did not dampen the celebratory tone of the graveside ceremony, held almost 111 years after Corporal John McBride's death.
The plaque commemorated the 31-year-old Irishman's three-year tour of duty for the Union army before he joined his younger brothers in Queenstown in 1863.
The wealthy businessman and farmer's ties to the United States were evident at the cemetery.
The plaque came courtesy of the American Civil War Round Table of Queensland and the Office of Veterans Affairs in Washington.
United States ambassador to New Zealand William McCormick attended, along with Territorial Force chaplain the Rev Anthony Harrison, and New Zealand Defence Force and Returned and Services Association representatives.
Director of Catholic Education for Otago-Southland and McBride descendant Tony Hanning, of Dunedin, welcomed several generations of John McBride's extended family and guests.
Mr Harrison, of Milton, gave readings and a blessing.
Choristers on tour from the University of Notre Dame in Indiana sang the two national anthems and joined the congregation in singing Oh Danny Boy, a recognition of John McBride's Irish roots.
Wreaths and floral tributes were laid and John McBride's grandniece, May Hanning, nee McBride, arranged for a piece of stone, from the original McBride family home in Ireland, to be placed on the grave.
The group adjourned to the reception at the Queenstown Events Centre, where land and buildings which were once part of the McBride farm could be seen.
Relatives from around the country met and worked out how they were related on the family tree.
Mr McCormick, from Portland, explained his personal interest in the history of the American Civil War and how many Irishmen were pressed into both the Union and Confederate armies as soon as they landed on American shores.
"There are numerous accounts of brothers fighting brothers in different uniforms," he said.
"John was right in the middle of our bloodiest war and when he left his battalion, he made a great choice in coming to New Zealand."
Mr McCormick gave a woollen Native American-styled blanket from his home state of Oregon to Mrs Hanning.
In return, she gave the ambassador a neck-tie with the tartan of County Antrim, in Northern Ireland, where John McBride was born in 1830.
Julie Marshall, nee McBride, of Wellington, described her childhood memories of visiting her aunts and uncles in the old farm house by Lake Hayes and spoke of the group's shared Celtic heritage.
