Mr. Sargent had long before this made up his mind to turn his attention to the law as a profession, and he accordingly began the study of the law at once with Hon. William P. Weeks, of Canaan, and remained with him till the spring of 1841, when he was advised by his physician to go South for his health. He went first to Washington, soon after to Alexandria, D. C., where he taught a high school, then to Maryland, where he remained a year in a family school, when, having regained his health, he returned to New Hampshire in September, 1842. He had, upon his arrival in Washington, entered his name as a law student in the office of Hon. David A. Hall of that city, and continued the study of the law under his direction, while engaged in teaching, and he was admitted to the bar in the courts of the District of Columbia in April, 1842, only about twenty months after leaving college. By the rule of that court any one might be admitted upon examination, without regard to the length of time he had studied; and he was examined in open court by Chief-Justice Cranch and his associates upon the bench, and was admitted.

After returning home, he continued his legal studies with Mr. Weeks until the July law term, in Sullivan county, in 1843, when he was admitted to the bar in the superior court of judicature in this state. He then went into company with Mr. Weeks at Canaan, where he remained till 1847, when he removed to Wentworth and opened an office there. He had been appointed solicitor for Grafton county in November, 1844, while at Canaan, and he at once commenced a lucrative business at Wentworth; was re-appointed solicitor in 1849 for five years more, thus holding the office for ten years, to 1854, performing the duties to the entire acceptance of the county and the people. He declined a re-appointment.

In 1851 he was first elected a member of the legislature from Wentworth, and served as chairman of the committee on incorporations. The next year he was re-elected, and was made chairman of the judiciary committee; and in 1853 he was again a member, and was nominated with great unanimity and elected speaker of the house of representatives. He served with ability and impartiality and to the general acceptance of the members. The next winter a new man was to be selected as a candidate for senator in his district, and he was nominated, and was elected in March, in a close district, by about three hundred majority. He was elected president of the senate in 1854. He was renominated in the spring of 1855, but the Know-Nothing movement that year carried everything before it, and he was defeated, with nearly all the other Democratic nominees in the state.

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.

Early in 1877 steps were taken for a revision of the statutes, and Judge Sargent was appointed chairman of a committee, with Hon. L. W. Barton of Newport, and Judge J. S. Wiggin of Exeter, to revise and codify the statutes of the state. Their work was completed and the statutes enacted by the legislature, to take effect the first of January, 1879. The volume was prepared and printed by the committee before the day appointed. It is the largest volume of statutes ever printed in the state, and it is believed not to be inferior to any other in any important particular.

In the fall of 1878, Judge Sargent was invited by a committee of the citizens of New London to prepare a centennial address, to be delivered on the one hundredth anniversary of the incorporation of the town. He at once accepted the invitation and set about the work, and on the 25th day of June, 1879, he delivered his address, and the occasion was distinguished by a larger collection of people, probably, than ever met in the town upon any former occasion. The address was published in the Granite Monthly, in the numbers for July, August, and September, 1879, and has been favorably noticed as a work of great labor and research.

Dartmouth College conferred on him the degree of Master of Arts, in course, three years after graduation; also, the honorary degree of Doctor of Laws, at its centennial commencement, in 1869. In compliance with a request from a committee of the trustees, he prepared and delivered at the commencement of 1880 at Dartmouth College a memorial address upon the late Hon. Joel Parker, formerly chief-justice of this state and afterwards professor of law in Harvard College. This duty Judge Sargent performed in a manner creditable to himself and satisfactory to the friends of the late Judge Parker. His address has been printed with other similar addresses in memory of other deceased judges, graduates of Dartmouth, by other distinguished sons of the college.