Born in Liguria, from a family of German origins. In 2010, Mager was disqualified for failing a doping test after smoking marijuana at a party.

He has won two ATP Tour singles titles, one an ATP 500 Series event in Rio in 2019, after which he made break-through into top 30, and the inaugural Forte Village Sardegna Open in 2020, an ATP 250 event. On 10 June 2019, Đere reached his career-high singles ranking of world No. 27. On 22 March 2021, he reached his career-high at world No. 346 in the doubles rankings. Đere debuted on the ATP Tour at the 2013 PTT Thailand Open, where he was a wild-card. His first attempt to play in main-draw at any grand-slams, happened on 2015 French Open, but his first successful attempt happened on 2016 French Open. At 2018 US Open, he marked his first grand-slam win, defeating Leonardo Mayer in first round. Laslo Đere was born on 2 June 1995 to mother Hajnalka and father Csaba (or Čaba) in Senta, Serbia. Both of his parents passed away from cancer. Đere began playing tennis at age 5 with his father. He also has one sister named Judit. He is fluent in Serbian, English, and Hungarian. His favorite surface is clay. His idols growing up were Andy Roddick, Lleyton Hewitt and Novak Djokovic. He is a supporter of the Chicago Bulls (NBA) and Seattle Seahawks (NFL).