There are several buses plying between Mumbai and Mahabaleshwar, which is a distance of about 247 kilometres. You will find both state transport as well as private buses from Mumbai and Pune. If you are going from Mumbai, you can catch the bus from Dadar East, Sion or Vashi. The ride will take you roughly six hours.
While Wathar (60 km) is the nearest railway station from Mahabaleshwar, the most accessible and convenient one would be at Pune. There are plenty of trains going to Pune from Mumbai as well as other major cities across India and you can take a bus or a taxi from Pune to Mahabaleshwar.
If you want to drive from Mumbai to Mahabaleshwar it will take you about 4 to 5 hours depending on the traffic. You have two options in terms of routes – one is to go from Panvel through Mahad and Poladpur; and the other option is to take the Mumbai -Pune expressway and the highway onwards from Pune to Satara. While the distance from the highway would be about 40 kilometres more but the ride is much better and you would actually reach faster.
Not sure of best route to Mahabaleshwar from your city. Search it on routeplanner-