Mark Ellis (born 4 February 1945 died 2025) was an American-Canadian naval architect, who designed sixteen production sailboats, along with many custom sailboats and powerboats. He is best known for his Nonsuch series of catboats, the Limestone series of powerboats and Niagara sailboats. In 2003, Dan Spurr described Ellis as "one of Canada's premier yacht designers".

Ellis was born in Watertown, New York, United States and lived a major portion of his adult life in Oakville, Ontario. He died in Essex, Connecticut United States and held dual American citizenship and Canadian citizenship.

Ellis grew up in upstate New York, where his father owned a department store. As a boy Ellis worked in the store and gained his first experience with business. He applied for a degree program in naval architecture at the University of Michigan but decided not to pursue it when he discovered that it mostly focused on large ship design, instead of small boats, which was where his interests lay. He went on to complete a degree in business administration at Boston University instead.

His family were sailors and Ellis learned to sail on an Atkin yawl, on Lake Ontario. His family went on to buy an island in the Thousand Islands and Ellis sailed there. He raced Dragon keelboats and became an instructor at the Clayton Yacht Club in Clayton, New York and the Crescent Yacht Club in Chaumont, New York.

Ellis started his career in 1965, at age 20 while attending university. His first job in the boat building industry was scraping the barnacles from boat bottoms. He liked to draw boat designs and described himself as a "doodler-draughtsman". He apprenticed at C. Raymond Hunt & Associates and, after graduating in 1968, under Phillip Rhodes. He quickly became a project manager for three custom boats being built in West Germany, using his business training to act as the liaison between the customer and the yard building the boats. With the birth of his first child Ellis and his wife decided to move away from New York City and so Ellis went on to work for Ted Hood at Little Harbour Boat Yard in Marblehead, Massachusetts, where he designed sailboat rigs as well as accommodation and deck plans. In 1970 Ellis and his family then moved to Canada where he worked in the design office at C&C Yachts under George Cuthbertson and George Cassian. At C&C he became a specialist in rig design, with his first assignment the C&C 61 Sorcery and second the C&C 43-1. Given his business background, Ellis gravitated to that end of the operation, noting that C&C did not have "a strong business orientation … boats would leave without being paid for, that sort of thing." Ellis ended up doing sales for C&C's custom division, the Bruckmann yard.

In 1975 C&C decided to expand, with new plants in West Germany and Rhode Island, despite an industry downturn underway. Ellis noted that he thought that business decision was more "eager than sense". Ellis decided to leave C&C and form his own design firm, Mark Ellis Design LLC, located in Oakville and later with offices in Essex, Connecticut.

Ellis' first design commission was the Aurora 40, but his second boat was the Nonsuch 30, a cruising catboat for customer Gordon Fisher. It became the first of what would become a large line of successful Nonsuch sailboats.

Ellis' background in business give him the knowledge and experience in writing contracts to retain control of his designs. Whereas many designers would design for a one-time payment for the rights, Ellis retained the rights to each of his designs and was paid for each individual boat completed. His contracts also gave him control of the tooling for the design, if the original manufacturer ended production or went out of business, allowing his designs to be more easily moved to a new manufacturer.