If your data set is left-skewed, then your mean will be less than your median.
In a left-skewed distribution, the tail of the distribution is longer on the left-hand side, which means that there are some extreme low values that pull the mean towards the left. On the other hand, the median is not affected as much by extreme values, since it only depends on the middle value(s) of the data set. Therefore, in a left-skewed distribution, the median is usually greater than the mean.