A staunch Federalist and nephew of President John Adams, Cranch moved his legal practice from Massachusetts to the new national capital, where he became one of three city land commissioners for Washington, D.C., and during his judicial service also was the 2nd Reporter of Decisions of the Supreme Court of the United States and a Professor of law at Columbian College (which later became George Washington University).

Cranch graduated in 1787, then read law with Thomas Dawes, a relative by marriage and judge of the Massachusetts Supreme Judicial Court.

[12] Nonetheless, the pecuniary troubles nearly caused Cranch to move back to Massachusetts, but he reconsidered after one of John Adams' final acts as President.

Cranch replaced Gustavus Scott and served for less than two months in 1801, trying to extricate the board from its lack of cash and general financial plight, while also continuing his vigorous private legal practice.

[13] On February 27, 1801, Congress passed the District of Columbia Organic Act of 1801, which among other things established the court system and Cranch became one of the city's first judges, leaving his role as commissioner.

[7] The court initially opened in Alexandria, Virginia (then part of the federal city) and after a courthouse was built in Washington, would alternate sessions between the locations.

[15] President John Adams nominated his nephew on February 28, 1801, to the United States Circuit Court of the District of Columbia.

[19] Moreover, on February 3, 1826, the Columbian College (now George Washington University) board of trustees selected Cranch and William Thomas Carroll, Esq., as that institution's first law professors.

On June 13 of the same year, President John Quincy Adams attended Professor Cranch's first law lecture, in the court room of the City Hall.

[21] Judge Cranch may today be best known for testifying in 1816 before a committee chaired by Rep. John Randolph of Virginia which investigated the practice of slavecatchers who kidnapped free Blacks in order to sell them further South as slaves.

[22] Judge Cranch was one of the initial members of the American Colonization Society later that year, and remained on its board of managers for decades.

In 1821, he held a trial in the case of William Costin (1780–1842), a free Black man whom a justice of the peace had convicted of refusing to both show his freedom papers and post a bond under a new law.

[24][25] In 1836, he ruled in favor of Isaac Carey, another free Black, who continued to sell perfume despite a new law prohibiting African Americans from working at any occupation other than involving transportation without a license.

[26][27] As a trial judge, Cranch heard several freedom suits by enslaved Blacks, many of them represented by attorney Francis Scott Key.