Born on 30 September 1679 to a family of merchants of Nantes, Cassard began a career as a sailor at age 14 on the merchantmen owned by his family. In January 1697, he joined the French Navy on bombship Éclatante.
In 1700, Cassard became a merchantman captain. The next year, the War of the Spanish Succession broke out, and Cassard converted to a privateer. In 1705, he captained the privateer Saint Guillaume, capturing 12 merchantmen and raiding Cork. Two years later, he captured 13 merchantmen with the Duchesse Anne, earning a rank in the Navy.
In 1709, Cassard, promoted to Commander, was tasked to escort a 25-ship food convoy on the 68-gun Éclatant. On 29 April, supported by Sérieux, he defeated five English ships, allowing the convoy to safely reach Marseille.
The next year, Cassard lead a squadron comprising the 74-gun Parfait, the 58-gun Sérieux and the 50-gun Phénix and the 60-gun Sirène, with his flag on Parfait. He was tasked to escort an 84-ship convoy inbound from Smyrna that had become blockaded at Syracuse by a British fleet. He captured HMS Pembroke, while Sérieux secured the surrender of the 32-gun HMS Falcon in the 9 November Battle of Syracuse. The convoy reach Toulon.
In 1711, Cassard again secured the way for a 43-ship convoy bound for Pensacola.
After the Treaty of Utrecht and the end of the war in 1713, Cassard started numerous trials to obtain payments.