Free attended Lincoln High School in Manitowoc, Wisconsin. As a senior, he was named Defensive Lineman of the Year, All-State, and All-Fox Valley Conference on both offense (tight end) and defense (defensive tackle).
He accepted a scholarship to Northern Illinois University with the plan of being converted to an offensive tackle. After being redshirted, he was forced to start at right tackle as a freshman, when sophomore tackle Shea Fitzgerald was killed along with 12 others, in a back porch collapse during a party in Lincoln Park, in Chicago, Illinois.
In college, he developed into an athletic player that earned him the nickname "Doug Freak" among his teammates. He eventually settled at left tackle and was named second-team All-MAC as a sophomore and first-team All-MAC and honorable mention All-American as a junior.
Free was selected in the fourth round (122nd overall) of the 2007 NFL Draft by the Dallas Cowboys. As a rookie, he was named a starter in his first preseason game, but suffered a knee sprain that would delayed his development and contributed to him being inactive for the first 15 games of the season, until playing at right tackle during the fourth quarter of the last game.
He was inactive during the first 13 weeks of the 2008 season. The next year, a season-ending injury to Marc Colombo pushed him into starting seven games at right tackle, performing at a high level while allowing only one sack. After the April 1, 2010 release of Flozell Adams, Free became the starting left tackle.
On July 26, 2011, the Cowboys avoided him testing the free agent market and rewarded him with a four-year, $32 million contract, with $17 million guaranteed. After his play regressed, the team decided to move him back to right tackle, exchanging positions with Tyron Smith in 2012. He was still seen as a liability even with the change, so the team gave more playing time to Jermey Parnell in the final four games of the season.
After having to share his playing time and giving up seven sacks, five holding penalties and seven false starts, he accepted a reduced salary in order to avoid being waived before the start of the 2013 season.
In 2014, he was a starter in what was arguably considered the best offensive line in the league and also emerged as the unit's leader. In October, he suffered a sprained right foot in the last minutes of the Week 6 win against the Seattle Seahawks, causing him to miss three games. He also missed the team's last four games (including two in the playoffs) with a left ankle stress fracture, that led him to have off season surgery.
On March 7, 2015, Free re-signed with the Cowboys on a three-year, $15 million contract. He started all 16 games and although he was a solid contributor at right tackle, he led the team in penalties (nine).
On March 11, 2017, Free announced his retirement from the NFL after 10 seasons.