He has been four times supervisor of Bedford and twice chairman of the Board of Supervisors. His legislative career began in 1848, when he was elected to the Assembly, and he was re-elected the following year. In 1853 he was chosen to the State Senate, where he at once took a prominent position. Among other public acts, he introduced the bill for establishing the Department of Public Instruction, which may justly be considered one of the most important events in the educational history of the State. In the Assembly of 1849, and also in the Senate of 1855, he supported Hon. W. H. Seward for United States Senator. Only one other person, Reuben Wells, of Warren, voted.;- twice for Mr. Seward for that office. In 1855 Mr. Robertson was elected county judge of Westchester County, and was twice re-elected to that responsible position, thus holding it for twelve years, and discharged the duties of the office with such ability and fairness as to win the commendation of the members of the bar, and merit the respect of all classes of citizens.

He served six years as inspector of the Seventh Brigade New York State Militia, was chairman of the military committee appointed by Governor Morgan in 1862 to raise and organize State troops in the Eighth Senate District, and was commissioned to superintend the draft in Westchester County.

In 1860 he was a member of the Electoral College, and voted for Abraham Lincoln. He supported him again in the National Convention of 1864, and during his whole administration was one of his most loyal and faithful adherents. In 1866 he was elected a representative in 'the Fortieth Congress by a majority of two thousand two hundred over William Radford, who had represented the district for the two terms immediately preceding.