Tim O'Brien has a sister and a brother. When he was twelve, the family moved to Worthington, Minnesota. The city is located on Lake Okabena in the western part of the state and will serve as the setting for some of his tales, especially in The Things They Carried.

O'Brien graduated in political science from Macalester College, where he was president of the student association, in 1968. That same year he was drafted into the US military and sent to Vietnam, where he served from 1968 to 1970 in the 5th Battalion of the 23rd Infantry Division. His division included the unit that was involved in the Mỹ Lai massacre.

After completing his service, O'Brien continued his graduate studies at Harvard University and completed an internship at the Washington Post. His writing career began in 1973 with the release of If I Die in a Combat Zone, Box Me Up and Ship Me Home, where he recounts his experience of war.

 He wrote there:

"Can the infantryman teach anything important about war just because he's been there?" I do not think so. He can tell stories of war. 

Although O'Brien insists that it is not his job to discuss the political reasons for the Vietnam War, he does occasionally make comments. Speaking years later of his upbringing and the war, he considers his hometown to be a city that welcomes, day after day, its own ignorance of the world: a city that has put us in Vietnam. The people of this city who sent me to this war […] could not spell the word "Hanoi" if you took away the three vowels. 

One characteristic of O'Brien's work is the indistinction between fiction and reality: his texts contain real details of situations he has experienced.

O'Brien lives in central Texas, where he is raising his young sons. He teaches at the University of Texas-San Marcos.