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.

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, He finished with 406 yards rushing on 56 carries, an average of 7.25 yards per carry, and six touchdowns.

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.”

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 he and Yeldon finished the year with over 1,000 yards each. In the Arkansas game, he had 55 rushing yards and three rushing touchdowns. Against Missouri, he had 177 rushing yards and three rushing touchdowns. In the Iron Bowl against Auburn, he had 131 rushing yards and two rushing touchdowns in the victory. He 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, he 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 made his NFL debut against the San Francisco 49ers and had 41 rushing yards and a rushing touchdown. He suffered a concussion against Washington Redskins on September 15, 2013. He returned against the Detroit Lions on October 6, 2013. He later rushed for 100+ yards in four games. He had a career-high in rushing in a single game against the Chicago 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. The Packers made the playoffs with an 8–7–1 record. In the Wild Card Round against the San Francisco 49ers, he had 81 rushing yards in the 23–20 loss.

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. In the second matchup against the Vikings, he had 125 rushing yards and a rushing touchdown to go along with two receptions for 13 yards and a receiving touchdown. In the season finale against the Detroit Lions, he had his third game of the season with at least 100 rushing yards.

Lacy started the 2015 season with 85 rushing yards and a touchdown in a victory over the Chicago Bears. On September 20, he 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, Lacy left during the 3rd quarter with a groin injury in Week 9’s game against the Carolina Panthers. On November 10, the Packers declared Lacy inactive for Week 10’s matchup against the Detroit Lions. On December 2, Lacy missed the Packers’ team curfew and his status was impacted for Week 13’s matchup against the Detroit Lions. On December 13, he had 124 rushing yards and a rushing touchdown in the 28–7 win over the Dallas Cowboys. Overall, in the 2015 season, he finished with 758 rushing yards, three rushing touchdowns, 20 receptions, 188 receiving yards, and two receiving touchdowns.

Lacy started the 2016 season with 61 rushing yards in a 27–23 victory over the Jacksonville Jaguars. On October 11, 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. He was placed on injured reserve on October 20, 2016. He started the first five games of the season, rushing for 360 yards on 71 attempts with no touchdowns and 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. He was offered similar contracts from the Packers 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.

Lacy struggled in the 2017 season as part of a crowded backfield. On September 10, in his Seahawks debut, he had five rushes for three yards in a 17–9 loss to his former team, the Green Bay Packers. Overall, he finished the 2017 season with 179 rushing yards and six receptions for 47 yards.