Lacy attended and graduated from Dutchtown High School in Geismar, Louisiana, where he played high school football for the Griffins. He ran for 1,207 yards and 13 touchdowns as a sophomore, 34 touchdowns as a junior, and one touchdown during his injury-plagued senior season. In December 2008, he was selected by the Louisiana Sports Writers Association as a first-team player on the Class 5A All-State football team for the second consecutive season. Rated as the 13th best running back and the 116th best prospect overall by Rivals.com, Lacy committed to the University of Alabama in February 2009.
Lacy was a highly touted recruit coming out of Dutchtown High School. He accepted an athletic scholarship to attend the University of Alabama, where he played for coach Nick Saban's Alabama Crimson Tide football team from 2009 to 2012. Lacy won three BCS National Championships with the Crimson Tide in his collegiate career.
With the Crimson Tide having a lot of depth at the running back position, including future NFL running backs Mark Ingram and Trent Richardson, Lacy was redshirted for his freshman year.
In 2010, Lacy earned his spot as the third-string tailback behind Ingram and Richardson. In the season opener against San Jose State, Lacy ran for 113 yards and two touchdowns in his collegiate debut. For the 2010 season, Lacy finished with 406 yards rushing on 56 carries, an average of 7.25 yards per carry, and six touchdowns. The 2010 season was Lacy's only one without winning a National Championship.
With Ingram departing for the NFL, Richardson and Lacy advanced to the No. 1 and No. 2 running back spots for the 2011 Alabama Crimson Tide football team. In the pre-season, those close to the program opined that "the Richardson-Eddie Lacy duo may put up bigger and better numbers and go down as the best duo in recent history." Backfield partner Richardson told reporters that Lacy's spin move is what separates him from other backs: "It's the nastiest spin move ever." The move has earned Lacy the nickname "Circle Button" based on the button on PlayStation's video games that triggers a spin move. Aaron Suttles of The Tuscaloosa News wrote, "To the fans, Eddie Lacy is a dreadlocked blur, spinning past defenders into the secondary. To his teammates, he's affectionately known as 'Circle Button.'"
In the 2011 season opener against Kent State, Lacy contributed 134 yards of offense, which were 76 receiving yards on three catches and 58 rushing yards on eight carries (an average of 7.3 yards per rushing carry). His 48-yard gain on a screen pass was the longest passing play of the game. Asked in a post-game press conference if he had hit the "Circle Button" a few times in the game, Lacy joked, "Yeah, I did a few spin moves." The Birmingham News wrote that Lacy "ran more impressively" against Kent State than the Crimson Tide's No. 1 running back, Trent Richardson. Another commentator led his coverage of the Kent State game with the headline, "Lacy, not Richardson, has look of Heisman-caliber RB in game one." After the game, head coach Nick Saban noted, "Eddie's a fast guy, an explosive guy." Photographs of Lacy hurdling over Kent State defensive back Calvin Tiggle were published in news outlets across the United States and in Europe, with one such image being chosen by Yahoo Sports as its lead "Photo of the Day."
In the second game of the 2011 season, Lacy rushed for 85 yards in Alabama's 27–11 win over Penn State.
In the third week, Lacy had the best game of his collegiate career, rushing for 161 yards and two touchdowns on nine carries against North Texas. His 67-yard touchdown sprint in the fourth quarter was picked as Alabama's Play of the Week.
Against Arkansas, Lacy rushed for 61 yards and one touchdown on 13 carries. He sustained a foot injury against SEC West rival Arkansas and did not play the following week against Vanderbilt.
During the 2011 regular season, Lacy had 631 rushing yards and seven touchdowns. His average of 7.5 yards per carry ranked sixth among all players in the NCAA Division I Football Bowl Subdivision.
In 2012, he became the starting running back for Alabama after the departure of Trent Richardson to the NFL. His backup was true freshman T. J. Yeldon and both Lacy and Yeldon finished the year with over 1,000 yards each. Lacy finished the 2012 season rushing for 181 yards and two touchdowns in the 2012 SEC Championship Game and 140 yards and one rushing touchdown, as well as one receiving touchdown in the 2013 BCS Championship Game against Notre Dame. He was named the Offensive MVP for the BCS Championship game. Lacy finished the year with 1,322 rushing yards with 17 rushing touchdowns and two receiving touchdowns. On January 11, 2013, Lacy decided to forego his final year and declared for the 2013 NFL Draft.
Lacy was one of the top running back prospects in the 2013 NFL Draft, with a reputation for agility, strength, and ability to run through tackles.
Lacy was selected in the second round (61st overall) by the Green Bay Packers in the 2013 NFL Draft. He was the fourth running back to be selected in the 2013 NFL Draft. In addition, he was the fourth of nine Alabama players to be selected that year. On May 30, 2013, he signed a contract with the Packers.
Lacy suffered a concussion against Washington Redskins on September 15, 2013. He returned against the Detroit Lions on October 6, 2013. In 2013, he rushed for 100+ yards in four games. He had a career-high in rushing in a single game against the Bears, running for 150 yards on 22 carries with a 6.8 average. Lacy passed the 1,000-yd season mark on December 15 against the Dallas Cowboys when he ran for 141 yards and a touchdown on 21 carries. Lacy finished the season with an impressive 1,178 yards and 11 touchdowns with 257 receiving yards. He set the Packer rookie records for most rushing yards and touchdowns in a season. He was named AP-Second Team All-Pro and was named to the NFC Pro Bowl.
Lacy was selected as the 2013 Offensive Rookie of The Year, the first Packer chosen for the award since running back John Brockington in 1971. Lacy also was the Packers' first 1,000 yard rusher since Ryan Grant in 2009.
The Packers 2014 season opener was against the defending champion Seattle Seahawks where Lacy had 12 carries for 34 yards and three receptions for 11 yards. Lacy sustained a concussion during the game and left the game in the second half. During the game, Lacy wore the new SpeedFlex helmet by Riddell, which claims that the helmet absorbs impact force better due to the helmet's greater flexibility. Lacy had his first 100-yard game of the season in the fifth game of the season against the Minnesota Vikings, where he had 13 carries for 105 yards, two touchdowns, and also had three receptions for 27 yards in a 42-10 Packers win.
After a slow start to the season, Lacy finished strong, ending the season with 1,139 yards rushing, with nine rushing touchdowns, averaging 4.6 yards per carry. Lacy became only the second Packer ever to rush for over 1,000 yards in his first two NFL seasons, joining John Brockington. In addition, Lacy caught 42 passes for 427 yards and four touchdowns.
On September 20, 2015, Lacy left Week 2's game against the Seattle Seahawks with an ankle injury and did not return. X-rays were negative on Lacy’s injured ankle. The injury was classified as "minor." On November 8, 2015, Lacy left during the 3rd quarter with a groin injury in Week 9's game against the Carolina Panthers. On November 10, 2015, the Green Bay Packers declared Lacy inactive for Week 10's matchup against the Detroit Lions. On December 2, 2015, Lacy missed the Green Bay Packers' team curfew and his status was impacted for Week 13's matchup against the Detroit Lions.
Lacy started the first five games of the season, rushing for 360 yards on 71 attempts with no touchdowns before being placed on injured reserve on October 20, 2016. On October 11, 2016, he injured his ankle in a Sunday Night Football game against the New York Giants. He had 81 yards on 11 carries before exiting the game. Lacy averaged a career-high 5.1 yards per carry in the 2016 season.
On March 14, 2017, Lacy signed a one-year, $5.5 million contract with the Seattle Seahawks with $3 million guaranteed. Lacy was offered similar contracts from the Packers, Seahawks, and Minnesota Vikings before ultimately choosing to sign with Seattle. On June 12, 2017, Lacy passed his latest weigh-in, which required him to be 250 pounds or less. He made $55,000 as part of his contract.