Mombasa to Amboseli NP

Entered Amboselli from the Namanga side, however drove through and exited onto the Tsavo Road. Didn't progress very far as were staying at the Sopa Lodge there. "Road" more of a dirt track - we actually smashed our bash plate off on the run in so be prepared for a typically rough african road. Severe corrugations on the western road in but doubt they'll be that bad on the eastern road as it is much less travelled.

Tsavo west roads are mapped but I have not driven them myself so cannot report on them. The Nrb / Mombassa road has reportedly been repaired at the Mombassa end so that should be "good tar" (well until the next rainy season that is), Athi river section still poor.

The main Mombasa - Nairobi highway is in good condition now that the Chinese have resurfaced it. However, the bit from West down to Amboseli (tarmac stops here!!!) is awful especially during the rains. I would break the journey up by staying at say Kilaguni Serena in West (which can take 6 or more hours) and then going onto Amboseli. Very often they give you a ranger as a guard for the bit between the two reserves.

I would drive from Mombasa to Emali approx 300 km (3-3/12 hrs, because Mombasa road is very good now) and than just before Emali turn left (there is a sign) to Amboseli NP. This last part is a dirt road is about 80 KM, which will take 2 hrs depending on the vehicle and weather.

I would agree with Mdadema with going over Voi - Emali route, especially if you are travelling during the wet seasons.
For your perusal I have linked a sight that might help with your plans, especially as it has a good range of Kenya maps.
http://www.webkenya.com/eng/safari/flash_map.php?tags02=Amboseli