The origins of the union lay in 1921, when Captain W. H. Coombes founded the Navigators and General Insurance Company Ltd.
It offered insurance for officers in the merchant navy against the possibility of the Board of Trade cancelling their certificate of competency.
A succession of small rivals began offering similar services, prompting Coombes to expand operations to the insurance of small vessels.
He also created a trust to administer the company, and this began representing officers in negotiations with employers and the government.
In 1956, the Marine Engineers' Association merged in, and the union was renamed as the MNAOA.