Born in New York City, he completed preparatory studies and became active with his brothers in a successful flour business which had been started by their father.
[1] He served on the board of managers of the Society for the Reformation of Juvenile Delinquents in the city of New York from 1845 to 1866, and was Treasurer from 1857 to 1866.
Underhill advocated an end to slavery, and also supported efforts to have former slaves settle in Liberia, with compensation paid to their former owners.
Regarded as a moderate on the slavery issue, Underhill frequently conversed with Alexander H. Stephens, later Vice President of the Confederate States of America, as they sought ways to end slavery which would obtain enough public approval to avert the coming American Civil War.
After leaving office Underhill was President of the Mechanics & Traders' Insurance Company in New York City.