Arnas was alumni of V.Knašius Basketball School at his young age, started his professional career at Neringa's Kuršiai in third Lithuanian league when he was 17. Next season he joined Sakalai in LKL. Arnas joined Neptūnas in 2013, after showing good performance in Lithuanian Basketball League playing for BC Lietkabelis scoring 8.1 points and 3.4 rebounds and 1.4 steals at his last season there, before the end of the season he joined Neptūnas for the playoffs. He quickly became an important part for the team, helping Neptūnas reach the LKL finals twice, as well as playing in the EuroLeague and EuroCup competitions.