Mompós is not easy to reach, but hardly impossible.
Bus Companies in Mompós – Copetran, Callejon del Cementerio No 3-74, 575-685-5768.
From the Caribbean Coast:
The fastest and most expensive way to get here is to fly into Cartagena or Barranquilla and organize an interurban taxi to Magangue. From there catch a motorized canoe over the river and then on from Bodega to Mompós.
Taxi, Cartagena to/from Magangue (they leave when full), 3-3.5 hours, $15-20.
Motorized canoe, Magangue to/from Bodega, 25 minutes, $3.
Taxi, Bodega to/from Mompós, 1 hour, $3.
Otherwise, Mompós has buses leaving every day to Barranquilla, Cartagena and Sincelejo (connections to Tolu, Covenas and Medellin). If you are stuck and have to leave, the best bet is to get to Magangue and try your luck there since there are more options.
Coming from the interior and Bogotá:
Omega and Copetrans has direct buses to El Banco (this is not wholly true since the buses do not reach El Banco). This bus (16–18 hours) leaves Bogotá at night and a drops travelers at the edge of a lake. The reason for this is that the long promised highway connecting to El Banco is being built through a swamp and has yet to actually connect to El Banco. At the water's edge one needs to hop into a motorized canoe ($2.50) to El Banco, then head to the market, roughly 25 meters away, and jump into a waiting 4x4 that runs the arduous 2-3 hour journey ($10-12) over bad roads to Mompós.
There are now direct minivans that run from Mompós to Cartagena costing $30-40 depending on the season.
River Travel:
From Barrancabermeja and other points on the Magdalena River there are infrequent departures to Magangue and El Banco. Be sure to ask around, as the river journey will be far more pleasant than that by road. The Colombian government has stated its intent on making the waterways in this region passable and hopefully soon there will be more available long distance options.