To find the scale factor of Mandy's drawing, we start with the given information: 1 millimeter on the drawing represents 1 meter in reality.
We can express this as a ratio:
\[ \text{Scale Factor} = \frac{\text{Drawing Length}}{\text{Actual Length}} = \frac{1 \text{ mm}}{1 \text{ m}} \]
To express this ratio in a standard form, we need to convert meters to millimeters. We know that:
\[ 1 \text{ m} = 1000 \text{ mm} \]
Therefore, we can rewrite the scale factor as:
\[ \frac{1 \text{ mm}}{1 \text{ m}} = \frac{1 \text{ mm}}{1000 \text{ mm}} \]
Now, simplifying this fraction:
\[ \frac{1}{1000} \]
Thus, the scale factor of the drawing is:
\[ \boxed{\frac{1}{1000}} \]