In January 1840, Fargo married Anna H. Williams (1820–1890), daughter of Nathan Williams, one of the proprietors of Pompey, with whom he had eight children:
Georgia Fargo (1841–1892), who died unmarried
Alma Cornelia Fargo (1842–1842), who died young
Sarah Irene Fargo (1843–1854)
William George Fargo, Jr. (1845–1872), who married Minerva Elizabeth Prendergast (1848–1873)
Hannah Sophia Fargo (1847–1851), who died young
Mary Louise Fargo (1851–1852), who died young
Helen Lacy Fargo (1857–1886), who married Herbert G. Squiers (1859–1911), a diplomat who served as Minister to Cuba (1902–1905) and Panama (1906–1909)
Edwin Morgan Fargo (1861–1865), who died young
In 1868, when he was 50, Fargo bought 5.5 acres (2.2 ha) on the Buffalo's west side and between 1868–1872, he built the Fargo Mansion at Jersey and Fargo Streets, which was Buffalo's largest mansion.
The home was completed in 1872 at a cost of $600,000 (equivalent to $12,805,000 in 2019).
Another $100,000 (equivalent to $2,134,000 in 2019) was spent to furnish and decorate the 22,170-square-foot (2,060 m²) mansion.
Michael Rizzo, a Buffalo historian, wrote:
the 'most elaborate and costly private mansion in the state,' outside of New York City.
The house took two city blocks, from Pennsylvania Avenue, West Avenue, Jersey Street, and Fargo Avenue.
There was a central tower five stories high.
At his request it contained wood from all the states of the Union.
It was the first home in the city to contain an elevator in it, and it was said to have gold doorknobs."

He died on August 3, 1881 after battling an illness for several months.
After his funeral on August 7, 1881, he was buried at Forest Lawn Cemetery.
At the time of his death, only two of his children were living, Georgia and Helen Fargo.
William's brother, J.C. Fargo, succeeded him as President of American Express after his death.
In January 1840, Fargo married Anna H. Williams (1820–1890), daughter of Nathan Williams, one of the proprietors of Pompey, with whom he had eight children:
Georgia Fargo (1841–1892), who died unmarried
Alma Cornelia Fargo (1842–1842), who died young
Sarah Irene Fargo (1843–1854)
William George Fargo, Jr. (1845–1872), who married Minerva Elizabeth Prendergast (1848–1873)
Hannah Sophia Fargo (1847–1851), who died young
Mary Louise Fargo (1851–1852), who died young
Helen Lacy Fargo (1857–1886), who married Herbert G. Squiers (1859–1911), a diplomat who served as Minister to Cuba (1902–1905) and Panama (1906–1909)
Edwin Morgan Fargo (1861–1865), who died young
In 1868, when he was 50, Fargo bought 5.5 acres (2.2 ha) on the Buffalo's west side and between 1868–1872, he built the Fargo Mansion at Jersey and Fargo Streets, which was Buffalo's largest mansion. The home was completed in 1872 at a cost of $600,000 (equivalent to $12,805,000 in 2019). Another $100,000 (equivalent to $2,134,000 in 2019) was spent to furnish and decorate the 22,170-square-foot (2,060 m²) mansion. Michael Rizzo, a Buffalo historian, wrote:
the 'most elaborate and costly private mansion in the state,' outside of New York City. The house took two city blocks, from Pennsylvania Avenue, West Avenue, Jersey Street, and Fargo Avenue. There was a central tower five stories high. At his request it contained wood from all the states of the Union. It was the first home in the city to contain an elevator in it, and it was said to have gold doorknobs."
He died on August 3, 1881 after battling an illness for several months. After his funeral on August 7, 1881, he was buried at Forest Lawn Cemetery. At the time of his death, only two of his children were living, Georgia and Helen Fargo. William's brother, J.C. Fargo, succeeded him as President of American Express after his death.
William Fargo