Born in Strafford, Vermont, Morrill attended the common schools, Thetford Academy and Randolph Academy. He was trained for a business career by working as a merchant’s clerk in Strafford and Portland, Maine. He then was a merchant in Strafford, and the partnership in which he participated with Judge Jedediah H. Harris grew to own and operate four stores throughout the state. Morrill also served in local offices including Town Auditor and Justice of the Peace.
One of Judge Harris's daughters married Portus Baxter, who also served in Congress. Baxter and Morrill became close friends as a result of the connection to Judge Harris, with Morrill referring to Baxter as "one of nature's noblemen" and Baxter consciously patterning his business and political career on Morrill's.
Morrill invested in several successful ventures, including banks, railroads and real estate. By the late 1840s he was financially secure enough to retire, and he became a gentleman farmer.
In addition to farming, Morrill became active in the Whig Party, including serving as Chairman of the Orange County Whig Committee, a member of the Vermont State Whig Committee, and a Delegate to the 1852 Whig National Convention.
In 1854 Morrill was elected to the Thirty-fourth Congress as a Whig. He was a founder of the Republican Party, and won reelection five times as a Republican, serving from March 4, 1855 to March 3, 1867. He served as chairman of the Committee on Ways and Means in the Thirty-ninth Congress. He also served on the Joint Committee on Reconstruction, which drafted the Fourteenth Amendment to the United States Constitution.
In 1866 Morrill was elected to the U.S. Senate as a Union Republican. He was reelected as a Republican in 1872, 1878, 1884, 1890, and 1896, and served from March 4, 1867, until his death, almost thirty-one years. He served as chairman of the Committee on Public Buildings and Grounds (Forty-first through Forty-fourth Congresses) where he played a vital role in obtaining the current Library of Congress main building through his work on the Joint Select Committee on Additional Accommodations for the Library. He also served as chairman of the Committee on Finance (Forty-fifth, Forty-seventh through Fifty-second, Fifty-fourth and Fifty-fifth Congresses). In addition, Morrill was a regent of the Smithsonian Institution from 1883 to 1898 and a trustee of the University of Vermont from 1865 to 1898.
The Morrill Tariff of 1861 was a protective tariff law adopted on March 2, 1861. Passed after anti-tariff southerners had left Congress during the process of secession, Morrill designed it with the advice of Pennsylvania economist Henry C. Carey. It was one of the last acts signed into law by James Buchanan, and replaced the Tariff of 1857. Additional tariffs Morrill sponsored were passed to raise revenue during the American Civil War.
Morrill is best known for sponsoring the Morrill Act, also known as the Land Grant College Act. This act was signed into law by Abraham Lincoln in 1862, and established federal funding for higher education in every state of the country. In his own words:
"This bill proposes to establish at least one college in every State upon a sure and perpetual foundation, accessible to all, but especially to the sons of toil, where all of needful science for the practical avocations of life shall be taught, where neither the higher graces of classical studies nor that military drill our country now so greatly appreciates will be entirely ignored, and where agriculture, the foundation of all present and future prosperity, may look for troops of earnest friends, studying its familiar and recondite economies, and at last elevating it to that higher level where it may fearlessly invoke comparison with the most advanced standards of the world."
—1862, as quoted by William Belmont Parker, The Life and Public Services of Justin Smith Morrill
He also authored the Morrill Anti-Bigamy Act of 1862, which targeted The Church of Jesus Christ of Latter-day Saints, based on the then-existing practice of plural marriage (polygamy). It imposed a five-hundred dollar fine and up to five years imprisonment for the crime of polygamy. On January 6, 1879, in Reynolds v. United States the Supreme Court, upheld the Anti-Bigamy Act's ban on plural marriage.
A second Land Grant College Act in 1890 targeted the former Confederate states and led to the creation of several historically black colleges and universities.
The Land Grant College Acts ultimately led to the founding of 106 colleges including many state universities, polytechnic colleges, and agricultural and mechanical colleges.
Morrill died in Washington, D.C. on December 28, 1898. He was buried at Strafford Cemetery.
At the time of Morrill's death his 43 years and 299 days of continuous Congressional service was the longest in U.S. history. He has since been surpassed, but still ranks 18th as of March 2014.
In 1851 Morrill married Ruth Barrell Swan (1822–1898) of Easton, Massachusetts. They had two children. Justin Harris Morrill (1853–1855) died in childhood. James Swan Morrill (1857–1910) graduated from the University of Vermont in 1880 and Columbian College Law School in 1882. He was a lawyer and farmer and served in a variety of offices including as a member of the Vermont House of Representatives.
The Justin Smith Morrill Homestead in Strafford is a National Historic Landmark.
Many colleges established under the Morill Act created a 'Morrill Hall' in his honor.
Morrill was initiated into the Delta Upsilon fraternity as an honorary member in 1864. He received honorary degrees from the University of Vermont, University of Pennsylvania, Dartmouth College, and many other institutions.
Justin Morrill College at Michigan State University was named for him.
In 1962, the U.S. Postal Service issued a 4 cent postage stamp to celebrate the centennial of the Morrill Land-Grant College Act. In 1999, the Postal Service issued a 55 cent Great Americans series postage stamp of Morrill to honor his role in establishing the land grant colleges.
In 1967 Ohio State University opened two residence halls on its campus. Named for Morrill and Abraham Lincoln, they are also known as The Towers. They are the tallest buildings on the OSU campus, and among the tallest in Columbus Ohio.