He was the son of James Denniston (ca. 1770–1825) and Prudence Morrison Denniston. On September 24, 1823, he married Julianna Howell (d. 1825). Afterwards he married Mary Scott, and they had five sons and six daughters.
He served as an officer of the New York State Militia and as a Justice of the Peace in Blooming Grove. He was appointed by Governor William L. Marcy a judge of the Court of Common Pleas of Orange County
He was a member of the New York State Senate (2nd D.) from 1841 to 1847, sitting in the 64th, 65th, 66th, 67th, 68th, 69th and 70th New York State Legislatures. In the Senate, he was Chairman of the Committee on Canals.
- The name is given sometimes erroneously, especially in contemporaneous newspaper accounts, as Robert Dennison
- Salisbury Mills is a hamlet that lies partly in the towns of Blooming Grove and Cornwall. Actually, he died at the same place where he was born, in that part of the hamlet of Salisbury Mills which lies within the Town of Blooming Grove.
-  Bio from History of The Town of New Windsor, Orange County, N.Y. by Edward M. Ruttenber (Newburgh NY, 1911, printed for The Historical Society of Newburgh Bay and the Highlands), at netcom
-  Obit from Albany Evening Journal, in NYT on December 8, 1867
-  Political Graveyard
-  Political Graveyard (doubled entry, giving surname as Dennison)
- Google Book The New York Civil List compiled by Franklin Benjamin Hough (pages 140, 269f; Weed, Parsons and Co., 1858)
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