Jankūnas played two seasons with the LKKA-Žalgiris, leading the team to a LKAL (Lithuanian League 2nd Division) title in 2003.

He was named the Lithuanian 2nd Division MVP after averaging 20.0 points, 12.3 rebounds and 1.5 blocks per game.

He was originally an early entry candidate for the 2005 NBA draft before withdrawing his name from consideration.

[4] However, Žalgiris Kaunas finished the season with a 2–8 record and did not qualify to the competition's Top 16 stage.

[5] Jankūnas was named the 2008–09 season's Baltic League MVP after averaging 15.4 points and 7.7 rebounds per game.

[6] On 14 August 2009, Jankūnas signed a two-year contract with the Russian EuroLeague club Khimki Moscow Region.

With Žalgiris Kaunas, he also won the Lithuanian Super Cup in 2012 and four Baltic League championships (2005, 2008, 2011, 2012).

[10] He played his last EuroLeague game on 3 April 2022, scoring 13 points in an overtime home victory over Crvena Zvezda.

Jankūnas was a member of the senior Lithuanian national team, for the first time at a major FIBA tournament, at the 2005 EuroBasket, where Lithuania finished in fifth place.