Immediately after winning admission to the bar, Daniel set up a legal practice in Boscawen, New Hampshire. He became increasingly involved in politics and began to speak locally in support of Federalist causes and candidates. After his father's death in 1806, he handed over his practice to his brother, Ezekiel, and opened a new practice in the larger town of Portsmouth. Over the decade-long period he lived in Portsmouth, he handled over 1700 cases, becoming one of the most prominent attorneys in New Hampshire. Along with two other lawyers, he was appointed to revise the New Hampshire criminal code and devise regulations for state prisons.