Aleksej Nikolić has previously played for Postojna, Zlatorog Laško, Spars Sarajevo, Bamberg, Baunach Young Pikes, Partizan, Treviso, Gravelines-Dunkerque, Burgos, Sassari, Brescia, Gran Canaria, and, before joining Cedevita Olimpija at the start of the 2024/25 season, he played for the French club Élan Chalon.

The Postojna native competed in the elite EuroLeague with Bamberg between 2015 and 2018, appearing in 51 games, starting 10 of them in the first five. In just over 10 minutes per game on average, he scored 2.4 points, dished out 1.4 assists, and grabbed 0.7 rebounds per game. In the EuroCup, before joining Cedevita Olimpija, he played for Partizan and Brescia. Nikolić also competed in the Basketball Champions League with Burgos and Sassari, playing a total of nine games. With Burgos, he averaged 6.2 points and 3.5 assists, and with Sassari, 7.3 points and 3.0 assists per game. In his career, Nikolić can boast titles as the Italian Cup champion in 2023 and the German Cup champion in 2017.

In the 2024/25 season, Aleksej won all three domestic titles with Cedevita Olimpija and was named the MVP of the Slovenian national championship final series against Krka. In four final series games, he averaged 14.5 points, 2.8 assists, and 3.1 rebounds per game. In the ABA League during the 2024/25 season, he played in 33 games, and in the EuroCup, he appeared in 20 games. In the EuroCup, he averaged 10.0 points, 3.7 assists, and 1.8 rebounds in just over 24 minutes per game, while in the regional league, he recorded an average of 10.5 points, 3.8 assists, and 2.0 rebounds per game.

With the Slovenian senior national team, he has competed in two World Championships (2014, 2023), two European Championships (2017, 2022), and the 2021 Tokyo Olympics. He has also been part of the national team in all recent qualification campaigns for major tournaments. Nikolić has been involved with Slovenian national teams since the U16 age category, with his greatest achievement being the 2017 European Championship title won with the Golden Generation of the Slovenian senior men’s team.