He enjoyed a diverse and successful career, and was sought for his designs of bridges, houses, monuments, and various commercial and public buildings.
Irish immigrants were at the time considered inferior, and during his early years Keller endured a considerable measure of hardship and discrimination.
Keller's association with Wight introduced him to the aesthetic philosophy of John Ruskin and to serious architectural study, which was cut short by the outbreak of the Civil War.
Though he won design competitions for Civil War monuments in several cities, his Soldiers and Sailors Memorial Arch at the entrance to Bushnell Park in Hartford, Connecticut, boldly broke the conventional form that had become the accepted configuration.
In contrast, Keller's Hartford monument, an eclectic Romanesque construction dedicated in 1886, was "perhaps the first permanent triumphal arch in the United States."
One of the arch's most striking elements is a bas-relief frieze featuring life-size figures carved by Bohemian-born sculptor Caspar Buberl.
Download coordinates as: and others Biographer David F. Ransom calls Keller's three small libraries "the crowning achievement of his career.