On April 2, 1855, he was appointed a circuit justice of the court of common pleas for the state. But in June of that year the old courts were abolished, mainly upon political grounds, and new ones organized, and new judges appointed. Judge Sargent received a request from Gov. Metcalf that he would accept the second place on the bench of the new court of common pleas. This offer had not been expected, but upon consultation with friends it was accepted, and Judge Sargent was appointed an associate justice of the court of common pleas. He acted as judge of the new court of common pleas for four years, until 1859, when, by a statute of that year, that court was abolished, and one new judge was to be added to the supreme judicial court, making the number of supreme judges six instead of five, as before. Judge Sargent was immediately appointed to that place on the supreme bench. He was then the youngest member of the court in age, as well as in the date of his commission. He remained upon the bench of that court just fifteen years, from 1859 to 1874. In March, 1873, upon the death of Chief-Justice Bellows, Judge Sargent was appointed chief-justice of the state, which place he held until August, 1874, when the court was again overturned to make room for the appointees of the prevailing political party. Chief-Justice Sargent, at the time of his appointment as chief-justice, had become the oldest judge upon the bench, both in age and date of commission. His written opinions are contained in the sixteen volumes of the New Hampshire Reports, from the 39th to the 54th, inclusive, numbering about three hundred in all. Many of these are leading opinions upon various subjects, and show great learning and research. After the repeal of the Missouri Compromise, and the attempt to make Kansas a slave state, Judge Sargent acted with the Republican party. Upon leaving the bench, in August, 1874, he was solicited to go into the practice of the law in Concord with William M. Chase, Esq., whose late partner, the Hon. Anson S. Marshall, had recently been suddenly removed by death. Judge Sargent accepted this offer, and thus at once stepped into an extensive and lucrative practice. This arrangement was made for five years. In 1876 he was elected a member of the constitutional convention of this state. In this convention he acted a prominent part. He was made chairman of the judiciary committee, the same place held by Judge Levi Woodbury in the convention of 1850. He took an active part in the debates and discussions of that body, and wielded an influence probably second to no one in the convention. He was also elected, by his ward in Concord, a member of the house of representatives for the years 1877 and 1878.