Daniel was admitted to the Middle Temple in May 1721 and to the Inner Temple three years later, and by 1731 he was settled in New York, where he was sworn attorney of the supreme court in March 1731/32. Having been "bred to the law, " he had strong backing in England and had brought letters to leading figures in the province. He promptly ranged himself with the governmental clique in New York politics and was soon rewarded by appointment to the council, September 29, 1733, to the office of recorder of New York City in 1736, and to that of third judge of the supreme court and admiralty judge in the same year.
In 1734 he began a service of thirty-eight years as vestryman of Trinity Parish. Apparently it was the influence of Chief Justice James DeLancey which was his chief reliance in his career as a courtier, for when DeLancey in 1746 turned the whole force of his far-reaching power in the province against Governor Clinton, Horsmanden was a conspicuous figure in "the faction. " In fact he was the writer of the portentous mass of labored communications from the Assembly. But as DeLancey's was the only commission granted "during good behavior, " Horsmanden was the easiest mark for the Governor's displeasure, and in 1747 he was stripped of all his offices.
His one avowed literary production was A Journal of the Proceedings in the Detection of the Conspiracy Formed by Some White People, in Connection with Negro and other Slaves, relating to the episode known as the Negro Plot of 1741. This was published in 1744, partly to justify the measures taken at the time, partly to rouse the citizens to feel a need for greater care in the regulation of the negro population, and partly, no doubt, for personal profit. By 1755 Horsmanden was restored to his seat in the council. He had in 1753 been reappointed to the supreme court and in 1763 reached the chief-justiceship, being obliged, however, to accept a commission running only "during pleasure. " This office he held until his death--several years after the infirmities of age had prevented him from rendering active service on the bench.
In 1765, as chief justice, he took exception to appeals from the supreme court to the governor and council on grounds of anything but error in law. The legal profession in the province was a unit in support of his position and the issue was skilfully used for political purposes. Horsmanden not only promoted popular agitation of the subject but by an ingenious use of technicalities succeeded in evading a direction from the King in Council to forward the record in a case. His last conspicuous public activity was as a member of the commission to inquire into the destruction of the Gaspee. He is said to have suffered indignities in the disorders of 1776.
His own death occurred in 1778 at Flatbush.
He married Mary Reade, the widow of Rev. William Vesey, the first rector of Trinity. His second wife was Anne Jevon.