Franklin G. Post founded his business in 1914. He had previously worked for the Lathrop Engine Company of Mystic where he became an expert machinist, specializing in the installation and repair of the developed gasoline engine.

However, he was determined to have his own yard, and eventually acquired property along the Mystic River's west bank. Here he dredged his own slip, and built a railway and boat shop. In 1923 he moved his expanding business to property across the river, a part of the new yard occupying the site of the old Irons & Grinell Shipyard. In its prime, Post employed as many as 80 workers, many of them skilled Nova Scotia boat builders.

During World War I the boatyard converted yachts to Coast Guard use, and repaired Coast Guard and fishing vessels. In World War II the shops turned out 45-foot aircraft rescue boats and harbor patrol boats for the U. S. Navy. Most of the business in the early days involved the Block Island fishing boats designed and built by the yard. More than 40 of these vessels were built and then became prototypes for Post's popular SEAGOIN hull, a company trademark for many years.

Perhaps the most famous Post-built vessel was the 62-foot auxiliary ocean cruising and racing Yacht ONKAHYA, launched in 1938. Designed by Sparkman & Stevens, the boat was built for Pemberton Whitecomb of New York. A 37-foot Mower-designed schooner also built by Post, made three Atlantic crossings. The MYSTIC ISLANDER, described by the Company as a "Husky, Roomy, Cruising Auxilary" (23' 6" L. W. L.) was developed in the mid 40's, and became one of Post's more successful products.

The Post yard also built a fleet of launches for Peru, and a dozen harbor tugs and personal boats of its own design. A fleet of 30-passenger lifeboats for the oil fields in Venezuela, and two flat bottomed launches for the jungle rivers of South America were also turned out. The biggest boats launched, were two speedy 80-foot cargo boats in the mid 1930's. These boats became well known during prohibition days for their activities beyond the 12-mile limit.

By the late 1940's Franklin's son, Ernest F. Post was President of the company, and during the 1950's the business became more involved in yacht brokerage activities including sales, service, and storage. In August 1959 the yard was sold and renamed Seaport Marine Inc., although Ernest Post retained the corporate name for his brokerage business.