In 1549, the "comunitas" of Bibbona ceded the estate of Cecina to the Tuscan Grand Duke's wife, Eleonora of Toledo. In the last years of the XVI century, the Fitto, an administrative palace near which a new wooden bridge and the foundry of Magona del Ferro were built, rose on demand of Ferdinando I de' Medici.

Cecina'e economy took off thanks to its belonging to the Tuscan Grand Duchy. The melting pot, which was set in a favourable position that allowed to get both timber for fire (through the "vincolo magonale" that forbade the inhabitants of nearby villages to cut the woods) and water, worked until the beginning of the XIX century.
In the XVII century, more productive activities linked to iron and steel production developed as well as breeding that were under the direct control of the Grand Duchy. Agriculture was penalized because its territorial extension diminished.
The estate of Cecina benefited from huge reclamation works that the Medici started and the Hapsburg-Lorraine, the successors of the Medici's ancient dynasty since 1737, finished. In 1739, the Grand Duke Francesco II ceded the territory of Cecina to the senator Carlo Ginori who began the real "colonization" of Cecina with his works. In effect, Ginori provided to the deforestation and the consolidation of a water network besides the recovery of vast parts of uncultivated lands.
The XIX century was the century when a dramatic population growth took place in Cecina and in its valley thanks to the finishing of reclamation works. Once the territory went back to the Grand Duchy it was divided into two parts. One was a hilly part destined to the supply of timber in the Magona. The other was a plain area that was sold to private investors after it was levelled between 1832 and 1846. This is how dozens of new farmhouses started to rise. The urban nucleus of the future town and the main market, instead, developed around the Fitto, the ancient Medicean palace.
At the end of the XIX century, the local economy totally changed with the closing up of the Magona del Ferro and because of a process of productive diversification that led Cecina's industry towards new activities in the food sector, above all.
Besides, since the twenties of the XX century, sea tourism developed. It caused the growth of Marina di Cecina and its linking path was traced in 1933.