Seth Thomas splits from partner Silas Hoadley in 1813 and sets up his own shop, and a few years later Eli Terry pays his former employee to produce a new style of shelf clock, called a "mantle". By the 1840s wooden gears are phased out as cheaper stamped-brass parts become standard. The factory has roughly 40 workers and annual production reaches 10,000 clocks in all models. 

The Seth Thomas Clock Company is officially incorporated in 1853 with $70,000. Spring-driven clocks are introduced, largely replacing weight-driven clocks, as annual production tops 30,000. Seth Thomas Sons & Co is formed in 1865 to make brass escapements and purchases an existing building for this purpose, which becomes known as the Marine Shop. By the 1870s the factory employs 500 workers turning out 100,000 clocks a year in over a hundred different models.

Work begins on an enormous addition to the Marine Shop for a new watch factory and automatic screw-cutting machines are purchased from the Star Manufacturing Co. New York City native Herman Reinecke is hired away from local rival Waterbury Watch Co in 1884 to design the Model 1, the factory's first watch, and is awarded two patents for it the following year when the 11-jewel gilt debut model is marketed to the public for the first time at a price of $12.00.  

Charles Higginbotham, master watchmaker at the Hampden Watch Co, succeeds Reinecke in 1886 and designs several new 18-size watches to replace the Model 1.  Higginbotham, born in Ireland in 1840 and a Civil War veteran, receives a patent for his improved mainspring T-end brace and would eventually become superintendent of the Illinois Watch Co.  Seth Thomas's Model 2, their first hunting movement, goes into production in four new grades, and the Model 3 succeeds the M1 as the factory's 18-size open-face watch, starting at SN 10,901. 

The male-stemmed hunting 6-size Model 14 is released as the firm's first ladies' watch in 1890.  Higginbotham is awarded another patent (his fourth one) for a new style of stem-setting mechanism , leading to the design of the 18-size Model 5 as Seth Thomas's flagship railroad pocket watch in 1893. The Maiden Lane arrives two years later as the factory's highest grade, named after the firm's New York City office, with a 17-jewel variant costing $25.00.      

The first Eagle Series (Models 6 and 7) is introduced as lower-cost open-face and hunting watches. The water tower built over the factory collapses just before Christmas in 1898, smashing through the roof into the basement, but none of the 175 workers gets injured. Higginbotham leaves for the Illinois Watch Co position in 1900 as production of the Models 2 and 3 ceases.  

The New Eagle Series (Models 8 and 9) is launched at a price of $3.65 each as the company's response to mass-produced "dollar" watches in 1904.  A new Trade Catalog is published, listing dozens of grades with parts lists for each model, and the key-wound Model 4 and the Old Eagle Series are both discontinued. The 0-size Model 20 is introduced as the factory's first wristwatch. 

The 16-size Model 21 begins production, and the Centennial Series is announced to commemorate Seth Thomas's 100th anniversary in a last effort to revive their fading pocket watch industry. A year later the supplemental catalog is published with only the Centennial Line remaining and pocket watch production ceases as the factory turns its efforts back to clocks.