The sentence, imposed Tuesday, is the harshest handed down so far in what is known here as the "parapolitica" scandal.
Garcia was "instigator and accomplice" of the Oct 16, 2000, massacre in the northern province of Bolivar, the court found.
On that date, 80 fighters from the North Bloc of the since-demobilized AUC militia federation clubbed and hacked to death at least 15 peasants in the village of Macayepo in the course of forcing some 200 families to abandon their homes and lands.
Garcia, an erstwhile political boss in the Caribbean province of Sucre who spent almost two decades in Colombia's Congress, has been held at a prison in Bogota since surrendering to authorities in November 2006.
The charges against him were based on testimony from demobilised AUC commander Jairo Castillo, who said the senator helped create militias in Sucre.
Fewer than half of the more than 60 politicos indicted in the parapolitica case have been convicted and sentenced, while the rest are awaiting trial.
Around 32,000 AUC fighters demobilised between 2003 and 2006 as part of a peace process with President Alvaro Uribe's government. The militia federation is blamed for more than 20,000 deaths.