Janis attended Saginaw Valley State University from 2009 to 2013, and played for the Cardinals. After red-shirting in 2009, he caught 9 passes for 130 yards and one touchdown in 2010. In 2011, he appeared in all 11 games, starting eight and posted a team-high 968 receiving yards on 48 receptions (20.2 avg.) and 14 touchdowns. In 2012, he started all 11 games and was second in the nation with 106 receptions and led all DII players with 1,635 receiving yards, both of which set new SVSU single season marks. He caught 17 touchdown passes and was named a first-team All-GLIAC selection. In 2013, he caught 83 passes for 1,572 yards and 14 touchdowns. He earned himself first-team All-GLIAC honors, and was a first-team AFCA Div. II All-American.

Janis was selected in the seventh round (236th overall) by the Green Bay Packers in the 2014 NFL Draft. Janis was the first Saginaw Valley State Cardinal drafted since offensive guard Todd Herremans in 2005, and the first wide receiver in school history to be drafted, though his early role on the team was more on the special teams rather than taking snaps at wide receiver.

Janis made his NFL debut on September 28, 2014 against the Chicago Bears, but he recorded no official statistics. In the next game against the Minnesota Vikings, he recorded two receptions for 16 yards. He appeared in one more game, which was against the Miami Dolphins, in the 2014 season.

In the 2015 season, Janis appeared in all 16 games in the regular season as a kick returner and backup receiver. His two receptions for 79 yards against the San Diego Chargers was his only receiving statistics for the regular season. He had a breakout game in the NFC divisional playoff game on January 16, 2016, against the Arizona Cardinals. He caught 7 passes for 145 yards and 2 touchdowns, including a 41-yard Hail Mary pass from Aaron Rodgers on the final play of regulation to bring the game into overtime. The Packers, however, lost 26-20 in overtime. 101 of these yards came on the game-tying drive to end regulation; he also caught a 60-yard pass on 4th and 20 from the Packers’ 4-yard line to set up the tying score.

On August 10, 2016, Janis fractured at least one of the bones in his right hand during a ball-security drill at Packers training camp. He remained in a backup role in the 2016 season. He had a receiving touchdown against the Atlanta Falcons in a 33–32 loss. Overall, in the 2016 season, he recorded 11 receptions for 93 receiving yards and a receiving touchdown in addition to a 19-yard rushing touchdown, which occurred against the Seattle Seahawks. In the 2017 season, he recorded two receptions for 12 yards and three kick returns for 43 net yards.

On March 30, 2018, Janis signed with the Cleveland Browns. On August 31, 2018, he was released.

In April 2019, Janis bought Timmy Tire Center in Oscoda, Michigan, where he had been employed in high school and college, and renamed it Janis Tire and Auto.