Collins attended South Plantation High School in Fort Lauderdale, Florida, where he was a letterman in football, basketball, lacrosse, and track. He was coached by Doug Gatewood. He was named Broward County Player of the Year as a junior after rushing for 1,786 yards and 28 touchdowns as the Paladins earned seven victories and qualified for the state playoffs. As a senior, he rushed for 1,400 yards and 14 touchdowns despite missing three games due to injury, helping South Plantation to nine wins, the district title and a berth in the 8A state playoffs. Following his senior season, he was selected to play in the Semper Fidelis All-American Bowl, where he was teammates with fellow Arkansas signee Austin Allen.
Also a talented sprinter, Collins ran the third leg on South Plantation’s state-champion 4 × 100 m relay (41.98 s), leading to him being named the 2013 Broward County Male Athlete of the Year.
Collins was ranked as a four-star recruit by Rivals.com and the 13th best running back in his class. Collins committed to play college football at the University of Arkansas in February 2013.
On signing day, Collins's mother provided some momentary drama, when she refused to sign the National Letter of Intent (LOI) for Alex to attend school at Arkansas, and then ran away with the LOI. She wanted Collins to stay close to home and attend classes at the University of Miami. The drama ended the next day when Arkansas head coach Bret Bielema faxed another LOI to Collins, which was signed by his father.
Collins started his career as the first true freshman in SEC history to rush for 300 yards in his first three games. Splitting time with Jonathan Williams, he finished the year with 1,026 rushing yards on 190 carries with four touchdowns. For his play he was named the SEC Freshman of the Year.
In his sophomore season, Collins ran for 1,100 yards and 12 touchdowns on 204 carries, while still splitting time with Williams. He also helped lead Arkansas to a Texas Bowl victory over the Texas Longhorns.
Williams went down with an injury before Collins' junior season, so he became the featured back in Bret Bielema's offense. On November 7, 2015, Alex became only the third player in SEC history to rush for 1000 plus yards in 3 seasons (Herschel Walker, Darren McFadden).
Collins finished his junior season with a career-high 1,577 yards and a school-record 20 rushing touchdowns. He was named second-team all-SEC, mostly because Alabama's Derrick Henry and LSU's Leonard Fournette outrushed him and were All-Americans. Collins led Arkansas to a Liberty Bowl victory over Kansas State on January 2, 2016. He finished the game with 185 yards and three rushing touchdowns, earning MVP honors.
Collins is second on the Razorback career rushing yards list (3,703), behind only Darren McFadden (4,590).
On April 30, 2016, the Seattle Seahawks selected Collins in the fifth round round, 171st overall, in the 2016 NFL Draft. On May 6, 2016, Collins signed a four-year deal worth $2.2 million overall with a $184,000 bonus. Collins finished his rookie season with 31 carries for 125 yards and 1 rushing touchdown, and catching 11 passes for 84 yards receiving, while appearing in 11 games for the Seahawks. Collins also had 8 carries for 27 yards and 3 receptions for 28 yards in two playoff games.
On September 2, 2017, Collins was waived by the Seahawks.
On September 5, 2017, Collins was signed to the Baltimore Ravens' practice squad. He was promoted to the active roster on September 16, 2017.