In 1763, he graduated from Harvard and began studying law in the office of Oxenbridge Thacher (died 1765), a top Boston attorney, whose practice he would take over in 1765.

A gifted orator, in 1766 he delivered an impassioned address in English "on liberty," or as others would recall it, on the meaning of being "a patriot," at Harvard's commencement upon receiving his Masters.

On February 12, 1770, he published in the Gazette a call to his countrymen "to break off all social intercourse with those whose commerce contaminates, whose luxuries poison, whose avarice is insatiable, and whose unnatural oppressions are not to be borne.

[1] After the Boston Massacre (March 5, 1770) he and John Adams defended Captain Preston and the accused soldiers and secured their acquittal.

[1] Prosecuting the case were Robert Treat Paine and Josiah's older brother Samuel Quincy, who shortly after was named solicitor general.