Dry dock work on the vessel is being conducted at the company's shipyard in Cartagena and, when completed, will give the submarine another five years of service life.
The S-73 is a diesel-powered vessel with a surface speed of 12 knots and a submerged speed 10.5 knots. It entered service with the Spanish navy in 1977.
The dry dock overhaul involved dismantling the submarine, replacing components and equipment in poor condition and then reassembling the vessel. Navantia said more than 15,000 pieces of equipment were removed and inspected, as well as its hull.
Mistral is docked at the shipyard for the remainder of the overhaul, which includes completion of assembly and testing at port and at sea.
The vessel is scheduled to be returned to the Spanish Navy in September, Navantia said.