Adlai Ewing Stevenson II was born in Los Angeles, California, in a neighborhood that is now designated as the North University Park Historic District. His home and birthplace at 2639 Monmouth Avenue has been designated as a Los Angeles Historic-Cultural Monument. He was a member of a prominent Illinois political family. His grandfather and namesake Adlai Stevenson I was Vice President of the United States under President Grover Cleveland from 1893 to 1897. His father, Lewis Stevenson, never held an elected office, but was appointed Illinois Secretary of State (1914–1917) and was considered a strong contender for the Democratic vice-presidential nomination in 1928. A maternal great-grandfather, Jesse W. Fell, had been a close friend and campaign manager for Abraham Lincoln in his 1858 US Senate race; Stevenson often referred to Fell as his favorite ancestor. Stevenson's eldest son, Adlai E. Stevenson III, became a U.S. Senator from Illinois (1970–1981). His mother was Helen Davis Stevenson, and he had an older sister, Elizabeth Stevenson Ives, an author who was called "Buffie". Actor McLean Stevenson was a second cousin once removed. He was the nephew by marriage of novelist Mary Borden, and she assisted in the writing of some of his political speeches.

Stevenson was raised in the city of Bloomington, Illinois; his family was a member of Bloomington's upper class and lived in one of the city's well-to-do neighborhoods. On December 30, 1912, at the age of twelve, Stevenson accidentally killed Ruth Merwin, a 16-year-old friend, while demonstrating drill technique with a rifle, inadvertently left loaded, during a party at the Stevenson home. Stevenson was devastated by the accident and rarely mentioned or discussed it as an adult, even with his wife and children. However, in 1955 Stevenson heard about a woman whose son had experienced a similar tragedy. He wrote to her that she should tell her son that "he must now live for two", which Stevenson's friends took to be a reference to the shooting incident.

Stevenson left Bloomington High School after his junior year and attended University High School in Normal, Illinois, Bloomington's "twin city", just to the north. He then went to boarding school in Connecticut at The Choate School (now Choate Rosemary Hall), where he played on the tennis team, acted in plays, and was elected editor-in-chief of The Choate News, the school newspaper. Upon his graduation from Choate in 1918, he enlisted in the United States Naval Reserve and served at the rank of seaman apprentice, but his training was completed too late for him to participate in World War I.

He attended Princeton University, becoming managing editor of The Daily Princetonian, a member of the American Whig-Cliosophic Society, a member of the Quadrangle Club, and received a B.A. degree in 1922 in literature and history. Under prodding from his father he then went to Harvard Law School, but found the law to be "uninteresting", and withdrew after failing several classes. He returned to Bloomington where he wrote for the family newspaper, The Daily Pantagraph, which was founded by his maternal great-grandfather Jesse Fell. The Pantagraph, which had one of the largest circulations of any newspaper in Illinois outside the Chicago area, was a main source of the Stevenson family's wealth. Following his mother's death in 1935, Adlai inherited one-quarter of the Pantagraph's stock, providing him with a large, dependable source of income for the rest of his life.

A year after leaving Harvard, Stevenson became interested in the law again after talking to Supreme Court Justice Oliver Wendell Holmes Jr. When he returned home to Bloomington, he decided to finish his degree at Northwestern University School of Law, attending classes during the week and returning to Bloomington on the weekends to write for the Pantagraph. Stevenson received his J.D. degree from Northwestern in 1926 and passed the Illinois state bar examination that year. He obtained a position at Cutting, Moore & Sidley, one of Chicago's oldest and most prestigious law firms.
Where did Adlai Stevenson II spend his early life?
Adlai Stevenson II was born in Los Angeles, California but raised in Bloomington, Illinois. After high school he moved to Normal, Illinois then went to boarding school in Connecticut. For college he went to Princeton then briefly attended Harvard Law School. After leaving Harvard he returned to Bloomington but attended classes at Northwestern University School of Law during the week.