Chennai to Mahabalipuram distance is about 50km (31miles). It is about 1 hour plus drive by road. You can take either of the two roads, the OMR (Old Mahabalipuram Road) or the ECR (East Coast Road / State Highway 49) to reach Mahabalipuram. The ECR as the name suggests runs along the coast. The travel distance for both the routes are comparable, the OMR route is a bit longer. The ECR is a toll road and preferred route between the two to reach Mahabalipuram. Both ECR and OMR roads proceed to Pondicherry which is about 100km (62miles) south of Mahabalipuram.
By the way you can catch a bus from Chennai's Koyambedu bus stand to Mahabalipuram. A taxi hire from chennai cost about Rs1700 including the waiting charges, toll fees and parking charges.
A milestone in Tamil language on the ECR showing distances to Pondicherry (101 km) and Mahabalipuram (5km)
See Also : Chennai to Mahabalipuram