Scammon was born in Whitefield, Maine. He came to Chicago when he was twenty-three. An attorney and a Whig, upon arriving in the city, he entered a legal partnership with Buckner Stith Morris, who was himself recently arrived from Kentucky. Their partnership lasted less than a year before Morris left the practice. In 1843, he served as the court reporter for the Illinois Supreme Court.
In 1844, Scammon founded the city's first newspaper, the Chicago Journal, a Whig-leaning newspaper that eventually became a Republican newspaper. Several years later, in 1861, Scammon sued the Democratic Chicago Democrat for libel after publisher John Wentworth published a cartoon which depicted Scammon as a wildcat banker. Scammon dropped the quarter million dollar suit only after Wentworth closed his paper, giving the subscription list to the Chicago Tribune.
Along with William Butler Ogden, he built the first railroad from Chicago in 1848, the Galena and Chicago Union Railroad, which ran from Chicago to a point ten miles west of town. When Eastern financiers refused to support the railroad, Ogden and Scammon raised the money by riding on horseback along the proposed route and taking donations from the farmers he passed.
Branching out, in 1851, Scammon founded the Marine Bank. He served as President of the Chicago Board of Education. He helped create Oak Woods Cemetery in 1854 and was the cemetery's fiurst president. In 1856, a group of men meeting in Scammon's law offices created the Chicago Historical Society.
Scammon was apparently active in the Underground Railroad, although he never publicly admitted as such. When he was accused of working to help slaves escape from law officers, he was asked what he would do if called upon to be part of a posse to capture fugitive slaves. Scammon replied, "I would certainly obey the summons, but I should probably stub my toe and fall down before I reached him."
In 1863, when the Chicago Astronomical Society determined to build an observatory affiliated with the University of Chicago, Scammon offered to pay for the construction of the observatory tower and dome as long as the observatory was named after his wife, Mary Ann Haven Dearborn. The group took him up on it and named the building the Dearborn Observatory. Scammon also paid the director's salary until he hit financial difficulties following the Great Chicago Fire in 1871.
In 1870, he donated the land and buildings for the Scammon Hospital, which was renamed following the Fire to the Hahnemann Hospital.
Scammon died in Chicago in 1890.