He enlisted in the Spanish navy and became a sailor at the age of 15 and saw his first action in an engagement at against a Barbary corsair.

During the Seven Years' War the British sent an expedition against Cuba in 1762 with a fleet of 23 ships of the line, 11 frigates, 4 sloops, 3 bomb ketches, 1 cutter alongside 160 troop transports, consisting of 31,000 men in total.

The Spanish forces opposing them had 11,670 men, 10 ships of the line, 2 frigates, 2 sloops and hundreds of cannons mounted on Havana's extensive fortifications.

10,000 soldiers disembarked under the command of George Keppel, 3rd Earl of Albemarle and captured the heights, which the governor of Cuba, Juan de Prado had left undefended.

The Earl of Albemarle allowed the transfer of the heavily wounded Velasco to Havana, where British surgeons tried to save his life, but in vain; he died on 31 July 1762.