Interracial marriage was illegal in Maryland before 1967, so in 1958 O'Brien's parents married in Washington, D.C., where marriage laws were less restrictive. The newly wedded O'Briens then moved to Long Island, to the town of St. James. O'Brien is the fifth of six children, all graduates of Harvard University. Her siblings are law professor Maria Hylton (born 1960), GE corporate Lawyer Cecilia Vega (born 1961), businessman Tony O'Brien (born 1962), who heads a documents company, eye surgeon Estela Ogiste (born 1964), and anesthesiologist Orestes O'Brien (born 1967).

O'Brien graduated from Smithtown High School East in 1984. She attended Harvard-Radcliffe College from 1984 to 1988, starting as pre-med and English and American literature, but left to take a job at WBZ-TV. O'Brien went back to school while pregnant with her first child and received her degree from Harvard University in 2000.

O'Brien began her career as an associate Producer and news Writer at WBZ-TV, then the NBC affiliate in Boston. She joined NBC News in 1991 and was based in New York as a field Producer for the Nightly News and Weekend Today. She then worked for three years as a local reporter and bureau chief for San Francisco NBC affiliate KRON. At KRON she was a reporter on "The Know Zone."

In 1995, O'Brien married Bradford "Brad" Raymond, co-head of investment banking at Stifel. They have four children: two daughters, Sofia (October 2000) and Cecilia (March 2002), and twin sons Charles and Jackson (August 2004).

Starting in 1996 and during the dot-com boom, O'Brien anchored MSNBC's weekend morning show and the cable network's Technology program The Site, which aired weeknights from the spring of 1996 to November 1997. The show was unique in that she interacted with a virtual character named Dev Null, played by Leo Laporte in a motion-capture suit.

From July 1999 to July 2003, O'Brien was co-anchor of the NBC News program, Weekend Today with David Bloom. During that time she contributed reports for the weekday Today Show and for weekend editions of NBC Nightly News. She also covered such notable stories as John F. Kennedy Jr.'s plane crash and the 1990s school shootings in Colorado and Oregon.

O'Brien moved to CNN, where from July 2003 to April 2007, she was co-anchor of the CNN program, American Morning CNN's flagship morning program that aired live from New York City.

In 2005, she covered the aftermath of Hurricane Katrina in New Orleans, where she interviewed then head of the Federal Emergency Management Agency (FEMA) Michael Brown.

O'Brien anchored a CNN special, Black in America, in July 2007. The program documented the successes, struggles, and complex issues faced by black men, women and families 40 years after the death of Martin Luther King Jr. In the first installment, O'Brien investigated how James Earl Ray, an armed robber and escaped convict, had already spent a year on the run just a month before his path collided with Dr. King in Memphis, Tennessee. In "The Black Woman & Family", O'Brien explored the varied experiences of black women and families and investigated the disturbing statistics of single parenthood, racial disparities between students, and the devastating toll of HIV/AIDS. The fifth installment of the Black in America series aired in December 2012.

In 2009, O'Brien completed a documentary titled Latino In America, documenting the lives of Latinos living in America. She continued working as a reporter for CNN, mainly hosting "In America" documentaries, and occasionally filled in for Anderson Cooper on Anderson Cooper 360. She also anchored exit poll coverage during CNN's coverage of the primaries and caucuses in the 2008 United States presidential race, and filled in for Paula Zahn on Paula Zahn Now before Zahn left CNN in 2007.

O'Brien has been riding horses since she was 13 years old, a hobby which she now enjoys with her family. She and her husband run a foundation called PowHERful Foundation (formerly called then Starfish Foundation, and before that the Soledad O'Brien & Brad Raymond Foundation), which mentors women to send them to college. The foundation began in 2011.

From January 2012 to March 2013, O'Brien was anchor of the CNN program, Starting Point. After CNN canceled American Morning and replaced it with two new programs, Early Start and Starting Point in 2011, O'Brien began anchoring Starting Point on January 2, 2012. It was announced February 21, 2013, that O'Brien was leaving Starting Point to start Starfish Media Group production company. March 29, 2013, was her last day on air at CNN as an anchor.

From 2013 to 2016, O'Brien was moderator of National Geographic Bee, replacing Alex Trebek who moderated for 25-plus years.

In 2014, O'Brien co-taught a Harvard University Graduate School of Education class with Professor Joe Blatt on "Advancing the Public Understanding of Education."

In 2016, O'Brien presented the 'I Am Latino in America' tour, with nationwide stops across the United States. The tour was streamed live globally on MOSH.