Admitted to the bar in Hillsborough County, New Hampshire, in September 1804, and in Plymouth County, Massachusetts that November, Shaw began practice in Boston. When his associate left Boston after being acquitted of murder in a political quarrel, he practiced alone for fifteen years.
In about 1822, Shaw took Sidney Bartlett, an able trial lawyer, as his junior partner. His practice gradually became large, but he was less known as an advocate than as the adviser of important commercial enterprises.
Shaw was prepared for his judicial career by numerous public positions. He was elected a member of the Massachusetts House of Representatives, serving in 1811-14, 1820 and 1829, and as a state senator in 1821-22. He served as a member of the constitutional convention of 1820. He also held many offices in Boston.
In 1822, with few precedents to guide him, he drafted the first charter of the city, which lasted until 1913. On the death of Chief State Justice Isaac Parker, Governor Levi Lincoln offered Shaw the appointment. Daniel Webster successfully urged Shaw to accept, though it meant giving up a practice of $15,000 to $20,000 a year for a salary of $3,500. And for this, if nothing else, Webster later thought the public owed him a debt of gratitude. Shaw's commission was issued August 30, 1830, and he served 30 years, resigning August 21, 1860.
His exceptionally long judicial career coincided with the development of many important industries, so that he made law on such matters as water power, railroads and other public utilities. Probably no other state judge has so deeply influenced commercial and constitutional law throughout the nation. Almost all the principles laid down by him have proved sound, although his remarkable skill in expounding the fellow-servant rule considerably delayed the replacement of that rule by workmen’s compensation.
An opinion by Shaw rarely lends itself to isolated quotations; its strength lies in the entire solidity of its reasoning. "His words had weight rather than brilliancy or eloquence",[1] and his greatness came from his personality as well as from his intellectual powers. He was no mere writer of opinions, but preeminently a magistrate.
In Shaw's time, the chief justice sat often at trials. In such work he was thorough, systematic, and patient, with a remarkable power to charge juries so that they understood the exact questions before them. Among his cases that excited great public interest were the trial in 1834 of the anti-Catholic rioters who destroyed the Ursuline convent in Charlestown.