Should I mention that I was an athlete in a curriculum for a PhD application? I am going to apply for a PhD in mathematics in north Europe. During my high school and undergraduate years, I was also a semi-professional athlete, and I retired when I was a graduate student to focus on my studies. I think being an athlete at a high level tells many things about a PhD student. A high level former athlete has probably developed a strong attitude for work, knows how to focus and how to manage their time; moreover, high level sport teaches you also how to manage stress and failures. However, I am afraid that a potential PhD adviser might get the wrong idea: I don't know any former athletes doing a PhD in my field, but I know at least one professor who thinks being an athlete could be a distraction. Unfortunately, in my country there is not a strong sports culture, and hence the academic and sport worlds are completely disjoint. Do you think I should mention my former career as an athlete in a curriculum for a PhD application?
I would put it in your CV. I see several reasons for this: first it completes your CV and you are 'honest' with anyone you apply to. I don't know the sports you are in but if someone googles your name, s/he might end up finding you which in my opinion raises questions (if you did not mention it).

The second reason is that of traits and skills a semi-professional career as athlete gives you: persistance, discipline and so on. Yes, it may distract you, but this is why you have chosen to focus on one path. Be prepared to answer questions about this when you are interviewed. Focus on the positive aspects that this occupation has given you.

Third: I think it highly depends on your sport. Is it athletics, football, or something else (popular) it is perhaps ok. Something like Mixed Martial Arts raises other questions and might put you into a unwanted light (agressive, conflict orientated). Sports like e-sports are also still a problem. Many do not recognise them as sports at all, although they might indicate team and communication skills as well as reflexes.

So I guess you should reflect upon the positive and negative aspects of your sport and why it has formed your character into the person you are know. If you can answer questions about this relation in an interview setting, you are pretty well off I think.