Kalnietis made his pro debut in February 2006 with the
Žalgiris Kaunas. He averaged 6.8 points on 65 percent shooting in the
EuroLeague. Kalnietis helped Žalgiris win a Baltic League title in 2008, two Lithuanian League titles in 2007 and 2008 and two Lithuanian Cups in 2007 and 2008. He also led the team to a 2007 ULEB Summer League title, where he was named MVP. He competed at the Reebok Eurocamp in
Treviso in 2006 and 2008. Kalnietis had a breakout season in 2008–09, when he averaged 11.3 points in the Lithuanian League and 8.5 points and 3.0 assists in the EuroLeague. He scored a career-high 20 points in a loss against the
Montepaschi Siena on 10 December 2008. Kalnietis signed a three-year contract with the
Benetton Treviso in June 2009. However, because he had another one-year contract with Žalgiris Kaunas, FIBA arbiter's decided that he belonged to Žalgiris. On 27 December 2010, he signed a three-year extension with his hometown club Žalgiris Kaunas. In October 2012, however, he announced that he's interested in playing abroad for the first time in Russia's
Lokomotiv Kuban Krasnodar. On 5 October, he signed a three-year contract with the team. On 18 December 2013, Kalnietis was named the
Lithuanian Basketball Player of the Year. On 27 April 2015, he parted ways with Lokomotiv. In July 2015, he joined the
Indiana Pacers for the
2015 NBA Summer League. On 17 July 2015, he returned to Žalgiris Kaunas, signing a one-year contract. On 23 January 2016, he left Žalgiris and signed a 1.5-year contract with
Olimpia Milano. On 24 July 2018, Kalnietis signed a one-year contract with
ASVEL of the French
LNB Pro A. In 2019, Kalnietis returned to the
PBC Lokomotiv Kuban. On 25 October 2020, Kalnietis scored 11 points, grabbed 4 rebounds, dished out 17 assists and broke the
VTB United League's all-time record of assists in a single game. On 21 June 2021, he has signed a two-year contract with
Žalgiris Kaunas, where his career had started. ==National team career==