As both Mars and Earth orbit the Sun (at different speeds) and both have slightly elliptical orbits this is not an easy question to answer because at some point Earth and Mars will be on opposite sides of the Sun and therefore very far apart while at other points they will both be on the same side of the Sun and therefore much closer together.
On average Mars orbits the Sun at a distance of 227,939,100 km
On average the Earth orbits the Sun at a distance of 149,597,888 km
Thus the distance between their "average" orbital paths is 78,341,212 Km. However, they would actually only be this distance from one another on rare occasions. Depending on where they are in their respective orbits, they can be anywhere from 36 million miles to over 250 million miles apart.