He was born on June 19, 1868, in Owosso, Shiawassee County, Michigan, the son of Henry Martyn Newcomb and Lucia (Turner) Newcomb. He attended the public high school in Washington, D.C.; and graduated from Williams College in 1892. Then he became a newspaper editor and worked for the New York Evening Post. He also studied law, was admitted to the bar in 1902, and practiced in New York City.
At the Republican primary for the New York state election, 1914, he supported Harvey D. Hinman for governor and, after Hinman's defeat, supported the Progressive ticket. In 1917, he managed Mayor John Purroy Mitchel's failed campaign for re-election.
- "J. Newcomb Dies, Ex-State Senator". New York Times. January 4, 1944. Retrieved 2014-12-29.
Counsel for National Utility Association in 1927-32. Once Newspaper Man
- New York Red Book (1902; pg. 149)
- Official New York from Cleveland to Hughes by Charles Elliott Fitch (Hurd Publishing Co., New York and Buffalo, 1911, Vol. IV; pg. 345, 349 and 367)
- Josiah T. Newcomb at RootsWeb