Armstrong redshirted his first year at Nebraska in 2012. He started his redshirt freshman year in 2013 as the backup to Taylor Martinez. Armstrong would end up starting eight games due to injuries to Martinez. Although he started those games, he mostly split time with Ron Kellogg. For the season he completed 68 of 131 passes for 966 yards with nine touchdowns and eight interceptions. He also rushed for 202 yards with two touchdowns. Armstrong started all 13 games as a sophomore in 2014. He completed 184 of 345 passes for 2,695 yards, 22 touchdowns and 12 interceptions and also rushed for 705 yards with 6 touchdowns. He returned as the starter his junior year in 2015. He completed 202 of 402 passes for 3,030 yards, 22 touchdowns and 16 interceptions. He also rushed for 400 yards and 7 touchdowns. He returned as the starter for his senior season in 2016 starting 11 of the 13 games missing 2 games with injuries. He completed 151 of 294 passes for 2,180 yards, 14 touchdowns and 8 interceptions. He also rushed for 604 yards and 8 touchdowns.

As a dual-threat quarterback, Armstrong sought to join the NFL by changing positions. Prior to the NFL Draft, Armstrong converted positions to wide receiver. Armstrong was not invited to the NFL Combine and, despite attracting interest during Nebraska’s pro day, was not drafted in 2017 NFL Draft. Armstrong was subsequently given a tryout with the Minnesota Vikings as a running back, but was later asked to sign as a safety, a defensive position and one he had not played since high school. Armstrong was not signed by Minnesota following his early May tryout but later did sign with the Vikings as a safety on May 30, 2017. He was later waived by the Vikings on June 16, 2017.

In May 2018, Armstrong signed with Your Call Football.

On September 9, 2018, Armstrong announced via his Twitter account that he had signed with the Salt Lake Stallions of the Alliance of American Football for the 2019 season. However, he went unselected in the 2019 AAF QB Draft.

On January 2, 2019, it was announced that Armstrong had signed with the Nebraska Danger, an Indoor Football League team based in Grand Island, Nebraska. Armstrong’s first game was a victory, in which he threw 8 completions on 12 attempts for 158 yards and 4 touchdowns, while also rushing for 23 yards and another score on 3 attempts. Armstrong was named the IFL Offensive Player of the Week for his performance in professional debut. Armstrong played in all 14 regular season games for the Danger, leading them to a playoff berth, finishing with a 7-7 record in 5th place. Armstrong completed 150 out of 251 passes, for 1,799 yards and 37 touchdowns against 9 interceptions, was the league leader in rushing yards and rushing touchdowns, with 137 attempts for 720 yards and 26 scores (both single season team records), and served as the holder for field goals and extra points. On June 21, 2019, Armstrong led the Danger to a 45-40 playoff win over the Green Bay Blizzard before suffering a 62-45 defeat at the hands of the Arizona Rattlers. During the playoffs, Armstrong completed 23 of 35 passes for 189 yards and 6 touchdowns against 2 interceptions, while also rushing for 63 yards and 4 more scores. Armstrong was named the Indoor Football League Offensive Rookie of the Year for 2019.

On October 17, 2019 Armstrong signed with the Sioux Falls Storm.