He started his pro career in 2009 with his hometown team BK Liepājas Lauvas. After a couple of seasons in Liepāja, he moved to Spain. His first team abroad was CB Clavijo from LEB Gold, where he played for two seasons. After the 2012-13 season, Mejeris went to the United States for NBA pre-draft workouts,[1] and although going undrafted, he performed well and earned feedback of being "two years from being NBA-ready".[2]
On July 15, 2014, he signed with VEF Rīga of the Latvian League.[4] Mejeris finished the season strong by helping VEF Rīga win the LBL championship while being named the MVP of the LBL playoffs.[5] In May 2017, Mejeris guided VEF to one more domestic championship, getting named Latvian League playoffs MVP once again.
On August 13, 2019, he signed with Parma Basket of the VTB United League.[6] In 2019-20 he averaged 10.2 points, 6.8 rebounds, 0.8 steals, and 0.8 blocks per game.[7] Playing for the team again in 2020-21, he averaged 9.0 points, 5.0 rebounds, 0.9 steals, and 0.9 blocks per game.[7]
On August 4, 2022, he signed with Hapoel Jerusalem of the Israeli Premier League.[9] On Hapoel Jerusalem, Mejeris was considered one of the team's best defenders. Mejeris was a significant member of the team that reached the Basketball Champions League finals, doing anything the coach requested, and always with a positive attitude.