Constantine M. Los was born on the Ottoman-controlled Greek island of Chios, in 1871. He was the youngest son of the shipping magnate Matheos S. Los. He was trained in maritime skills in Chios. In 1904 he went to New York and became the chief engineer of a line of ferryboats operating in New York harbour, and in 1906 he went to Panama to assist in the maritime aspects of the building of the Panama Canal.

Around 1910 he returned to Chios. Utilizing the personal capital he amassed in the Americas, he assumed control of the fleet his father left to his older sons after his death in 1881.

Between 1914 and 1930 he revamped the Los fleet, replacing several ships with the then modern steam ships. He continued to expand the fleet until the outbreak of World War II.

In 1922, as the Turkish army invaded Asia Minor westward towards Smyrna (the Destruction of Smyrna), Costis Los offered and personally captained his only locally available ship to the Greek efforts to assist the 1,500,000 resulting Greek refugees by transporting them to safety in mainland Greece.

During World War II, Costis Los' entire fleet was requisitioned by the Allied Forces, and in turn suffered losses. By the end of the war, his sons had established offices in London, Montreal and New York, although he continued to oversee operations from Chios.

The New York-based sons had played a role in the decision of the American government to liquidate its wartime fleet of Liberty Ships to private interests that had lost ships on behalf of the Allied Forces during the war, including the allocation of 107 of these ships to Greek shipowners, with lucrative sale terms.

With the profits of the Liberty Ships allocated to Costis Los, the fleet began to grow once again, and by the 1950's he was commissioning new shipbuildings in Japan.

Costis Los is also credited with numerous charitable works in Chios, including the construction of a water-pumping station and the development of a beach, now called Los Beach.

Costis Los died in 1962 in Chios, having already passed the operation of his fleet to his sons.