L am going to echo Swordfish and note that there is a semantics problem with this question. So I am not going to count some early tease & denial experiments nor my first "spanking".

My first real mistress was named Heather and she worked in a CT "health spa" where some laws were broken. At first I went there for vanilla but when I expressed some interest in some S&M one of the ladies said Heather was the establishment's specialist in that. So I sought her out next time. Heather was quite intense both in terms of pain and domination. Sometimes the lady in an adjourning room would yell out something like "tone it down, Heather!"