He took part in the 1920 Summer Olympics, organized in his native county, and in the 1924 Summer Olympics. In the 1920 singles event he won the first round against Hans Syz, followed by a victory over Carl-Eric von Braun in the second round but in the third round Ichiya Kumagae proved too strong. In the 1920 doubles event Lammens partnered with Jean Washer. They defeated Sune Malström and Carl-Eric von Braun in the first round but lost in the second round in straight sets to the Japanese pair Ichiya Kumagae and Seiichiro Kashio. He played in the mixed doubles together with Anne Chaudoir and after a bye in the first round and a win in the second against Jeanne Vaussard and François Blanchy, they were defeated in the third round in three sets by the eventual Olympic champions Suzanne Lenglen and Max Decugis.