In 1866, he opened the Merchants' Bank of Watertown. In April 1870, the bank was incorporated as a stock company, and he became chief executive officer of the company. In 1869, he bought a cotton factory in Factory Square in Watertown, and he started to heavily improve the square. The Carthage, Watertown and Sackets Harbor Railroad started to be built in 1872, and Winslow was one of the directors. In 1873, he built the Winslow Block, a building in the Public Square in Watertown. He built almost 100 buildings in the southeastern part of Watertown. 