Is "on your mark" a common expression?
Yes. I believe it is a common expression in the United States. It is used as a command to start a race, especially for children. In Great Britain, they typically say, "Ready, steady, go!" Even though these expressions sound different, both mean the same thing. Here are some examples of both being used: Ex: Okay, so we race down the hill and whoever is first wins. On your mark. Get set. Go! = Okay, so we race down the hill and whoever is first wins. Ready, steady, go! Ex: I'm going to win. On your mark, get set, go! = I'm going to win. Ready, steady, go! Either one of these expressions are totally fine to use. Thanks for your question!