John Lowell Jr. was the son of John Lowell (1743–1802). He graduated from Phillips Andover and then from Harvard in 1786. After studying law, he was admitted to the bar in 1789 (like his father, before he was twenty years old). In 1797, he gave the Fourth of July Address at Faneuil Hall in Boston, in which he criticized revolutionary France. From 1798-1801, he served as a Federalist member of the Massachusetts legislature. In 1801, along with Harrison Gray Otis, he defended Jason Fairbanks against charges of murdering Elizabeth Fales in Dedham, Massachusetts. Despite a vigorous defense, Fairbanks was convicted and executed by hanging. This, followed by the death of his father in 1802, caused him to suffer a breakdown. He retired from active law practice in 1803, and traveled with his family to Europe for the next three years, touring England, France, and Italy.