In 1814, in the midst of his political career, he was also named a judge of the court of common pleas (1814–1818), and played a leading role as delegate to the controversial Hartford Convention in which New England's secession from the United States was discussed.

He did so, and also quietly arranged his own private purchase of 18.5 acres (75,000 m2) adjoining from the agent of John Singleton Copley, then living in England.

After a decade of legal arguments, the sale was upheld, and Otis and the Mount Vernon Proprietors developed a large part of Beacon Hill.

Together, Harrison and Sally were the parents of eleven children, including:[5] He died in Boston on October 28, 1848, and is buried in Mount Auburn Cemetery, Cambridge, Massachusetts.

[5] Through his son William, he was the grandfather of Emily Marshall Otis (1832–1906), who married historian and educator Samuel Eliot.