He played college football for the Iowa Hawkeyes and was selected by the Detroit Lions 23rd overall in the first round of the 2012 NFL draft.

Born in Parkston, South Dakota to Jo and Tom Reiff, Riley attended Parkston High School, where he was a two-time South Dakota Elite 45 team selection under head coach Jon Mitchel.

During his high school career, Reiff recorded 261 total tackles, 23 sacks and eight interceptions on defense, and also caught 27 passes for 321 yards and nine touchdowns on offense.

In addition to football, Reiff also lettered in golf, track and was a three-time wrestling champion with a career record of 121–1, claiming three state titles.

He later had second thoughts and switched his verbal commitment to Nebraska, indicating that he "pulled the trigger a little too early" with Iowa, but when the Cornhuskers started sinking under the direction of Bill Callahan, Reiff found his way back to the Hawkeyes.

In his sophomore year, Reiff replaced Bryan Bulaga at left tackle, starting all 13 games of the season.

He was named All-Big Ten by coaches and media, as well as All-American by Pro Football Weekly, which evaluates players on NFL prospects and draft value rather just college production.

[9] At the combine, Reiff displayed decent athleticism but disappointed with his measureables; his 33+1⁄4 in (84.5 cm) arms were deemed short for an NFL left tackle.

Reiff had one start at the left tackle position coming in the teams Thanksgiving Day matchup against the Houston Texans.

Despite his limited role in the 2012 season, Reiff was rated as the Lions top run blocker by Pro Football Focus (PFF).

[18] He started at left tackle in the season opener game against Minnesota on September 8 and helped the Lions gain 481 yards of total offense.

In 2014, Reiff started his mainstay at left tackle in Week 1 against the New York Giants on September 8 and provided protection that led to 20 first downs for the Lions offense.