A combo guard, Janning attended Watertown-Mayer High School, where he averaged a Metro-high 31.8 points per game and 8.4 rebounds, earning All-Conference honors three times, a two-time All-Area award and was a two-time team MVP.
After high school, Janning played college basketball with the Northeastern University Huskies. He was proclaimed as one of Northeastern University's most prolific scorers of the program after ending as the program's fourth all-time leading scorer with 1,836 points. He was a three-time All-CAA performer, including first-team honors in 2009 and 2010.
After going undrafted in the 2010 NBA draft, Janning played with the Boston Celtics in the Orlando Summer League and later for the Phoenix Suns in the Las Vegas Summer League. On August 4, 2010, he signed a multi-year contract with the Phoenix Suns. On November 16, he was released by the Phoenix Suns before appearing in a game for them.
On December 1, he was signed by the Maine Red Claws, after they released Jordan Eglseder, due to injury. On January 20, he was traded to the Rio Grande Valley Vipers, in exchange for Antonio Anderson. He led the Rio Grande Valley to the NBDL Finals, averaging 11 points per game.
On July 16, he signed a 1-year contract for $120,000 with Novipiù Casale Monferrato of the Lega Basket Serie A. In his first Italian League season, he averaged 12.6 points, 3.6 rebounds, and 1.7 assists per game, while shooting 41% from 2-point range, 38% from 3-point range, and 70% from the free throw stripe. Janning then played with the Indiana Pacers in the Orlando Summer League, and later for the Memphis Grizzlies, in the Las Vegas Summer League, during the 2012 summer.
Janning then signed with the reigning Italian League champions, Montepaschi Siena, in August 2012. He made the Lega Basket Serie A BEKO All-Star team. In his 2nd Italian League season he is averaging 7.1 points, 2.3 rebounds, and 1.0 assists per game, while shooting 48% from 2-point range, 31% from 3-point range, and 87% from the free throw stripe.
On August 4, 2013 it was announced that Janning signed for Croatian champions Cibona Zagreb. He was released in December 2013, due to financial problems of the club. On January 3, 2014, he signed with his former club Montepaschi Siena for the rest of the season.
On July 10, 2014, he signed a 1+1 contract with the Turkish team Anadolu Efes.
On September 28, 2015, Janning signed with the Denver Nuggets. However, he was later waived by the Nuggets on October 15 in order for him to sign with Hapoel Jerusalem of the Israeli Basketball Premier League. On February 7, 2016, he parted ways with Hapoel Jerusalem. Two days later, he signed a deal with Lokomotiv Kuban until the end of the 2016–17 season.
On September 15, 2017, Janning signed a two-month deal with Baskonia of the Liga ACB and the EuroLeague to replace the injured Jordan McRae.
Note: The EuroLeague is not the only competition in which the player participated for the team during the season. He also played in domestic competition, and regional competition if applicable.