Trained as a lawyer, he served in the provisional government of Massachusetts during the American Revolutionary War, and was elected to the Confederation Congress in 1782.

[1][2] The elder Increase Sumner was a successful farmer descended from early settlers of Dorchester; he held a number of public offices including coroner for Suffolk County, and selectman of Roxbury.

[2] Sumner excelled at school, and over the resistance of his father (who envisioned his son's future to be in agriculture) was enrolled at Harvard College in 1763.

[4] After graduating from Harvard, Sumner took charge of the Roxbury school, where he taught for two years while he apprenticed law under Samuel Quincy, the provincial solicitor general.

[6] In June 1782 he was elected to the Confederation Congress by the state legislature, replacing Timothy Danielson, who resigned, but Sumner never actually took the seat.

In August 1782 Governor John Hancock nominated him as an associate justice of the Massachusetts Supreme Judicial Court to replace James Sullivan.

These economic pressures led to outbreaks of civil unrest which culminated in Shays' Rebellion, an uprising in central and western Massachusetts lasting from 1786 to 1787.

[23][24] On June 2 Sumner rode from his home in Roxbury accompanied by 300 citizens on horseback to the State House in Boston, where the Secretary of the Commonwealth proclaimed his governorship from the eastern balcony.

[27] During Sumner's period in office the state was principally preoccupied with the threat of attack by France as a result of the ongoing naval Quasi-War.

Comparatively younger and more vigorous than his predecessors, Sumner actively built up the state militia and worked to ensure its preparedness in case of attack.

[29] The funeral procession which included four regiments of militia ran from the governor's Roxbury mansion to a service at the Old South Meeting House.

He was for sometime a practitioner at the bar; and for fifteen years an associate judge of the supreme judicial court; was thrice elected governor of Massachusetts in which office he died.

Calumny was silenced by the weight of his virtues and rancour softened by the amenity of his manners in the vigour of intellectual attainments and in the midst of usefulness.

[41] At his confirmation hearings in 2017, U.S. Supreme Court Justice Neil Gorsuch recalled being moved by reading Sumner's gravestone as a law student at Harvard.

Gorsuch closed his opening statement by reading a portion of Sumner's epitaph and adding "[T]hose words stick with me.

They serve for me as a daily reminder of the law's integrity, that a useful life can be led in its service, of the hard work it takes, and an encouragement to good habits when I fail and when I falter.