Though he was an avid reader who fell in love with scientific and mathematical theories, he did not do well in science classes in college, and avoided taking math. His grades during his first two years put him in the lower one-fifth of the class. During his sophomore year, he reportedly spent much of his time watching television, shooting pool, and occasionally smoking marijuana. In his junior and senior years, he became more involved with his studies, earning As and Bs. In his senior year, he took a class with oceanographer and global warming theorist Roger Revelle, who sparked Gore's interest in global warming and other environmental issues. Gore earned an A on his bachelor of arts thesis, "The Impact of Television on the Conduct of the Presidency, 1947-1969", and graduated with honors in June 1969.

Gore attended college during the era of anti Vietnam War protests. Though he was against that war, he disagreed with the tactics of the student protest movement, thinking it silly and juvenile to take anger at the war out on a private university. He and his friends did not participate in Harvard demonstrations. John Tyson, a former roommate, recalled that "We distrusted these movements a lot .... We were a pretty traditional bunch of guys, positive for civil rights and women's rights but formal, transformed by the social revolution to some extent but not buying into something we considered detrimental to our country." Gore helped his father write an anti-war address to the Democratic National Convention of 1968, but stayed with his parents in their hotel room during the violent protests.

When Gore graduated in 1969, his student deferment ended and he immediately became eligible for the military draft. His father, a vocal anti-Vietnam War critic, was facing a tough reelection fight in 1970. Gore eventually decided that the best way he could contribute to the anti-war effort was to enlist in the Army, which would improve his father's reelection prospects. Although nearly all of his Harvard classmates avoided the draft and service in Vietnam, Gore believed if he found a way around military service, he would be handing an issue to his father's Republican opponent. According to Gore's Senate biography, "He appeared in uniform in his father's campaign commercials, one of which ended with his father advising: 'Son, always love your country.' Regardless, Al senior lost his seat.

Gore has said that his other reason for enlisting was that he did not want someone with fewer options than he to go in his place. Actor Tommy Lee Jones, a former college housemate, recalled Gore saying that "if he found a fancy way of not going, someone else would have to go in his place." His Harvard advisor, Richard Neustadt, also stated that Gore decided, "that he would have to go as an enlisted man because, he said, 'In Tennessee, that's what most people have to do.'" In addition, Michael Roche, Gore's editor for The Castle Courier, stated that "anybody who knew Al Gore in Vietnam knows he could have sat on his butt and he didn't."

After enlisting in August 1969, Gore returned to the anti-war Harvard campus in his military uniform to say goodbye to his adviser and was "jeered" at by students. He later said he was astonished by the "emotional field of negativity and disapproval and piercing glances that ... certainly felt like real hatred".

Gore had basic training at Fort Dix from August to October, and then was assigned to be a journalist at Fort Rucker, Alabama. In April 1970, he was "Soldier of the Month".

His orders to be sent to Vietnam were "held up" for some time and he suspected that this was due to a fear by the Nixon administration that if something happened to him, his father would gain sympathy votes. He was finally shipped to Vietnam on January 2, 1971, after his father had lost his seat in the Senate during the 1970 Senate election, becoming one "of only about a dozen of the 1,115 Harvard graduates in the Class of '69 who went to Vietnam." Gore was stationed with the 20th Engineer Brigade in Bien Hoa and was a journalist with The Castle Courier. He received an honorable discharge from the Army in May 1971.

Of his time in the Army, Gore later stated, "I didn't do the most, or run the gravest danger. But I was proud to wear my country's uniform." He also later stated that his experience in Vietnam "didn't change my conclusions about the war being a terrible mistake, but it struck me that opponents to the war, including myself, really did not take into account the fact that there were an awful lot of South Vietnamese who desperately wanted to hang on to what they called freedom. Coming face to face with those sentiments expressed by people who did the laundry and ran the restaurants and worked in the fields was something I was naively unprepared for."

Gore was "dispirited" after his return from Vietnam. NashvillePost.com noted that, "his father's defeat made service in a conflict he deeply opposed even more abhorrent to Gore. His experiences in the war zone don't seem to have been deeply traumatic in themselves; although the engineers were sometimes fired upon, Gore has said he didn't see full-scale combat. Still, he felt that his participation in the war was wrong."