According to the Wisconsin Historical Society, the Town (and village) of Merrimac was named by Mrs. J.G. Train for the county of Merrimack in New Hampshire. The original spelling included the letter k, the same as the New Hampshire name. The village of Merrimack was originally called Matts ferry for Chester Mattson who operated a ferry there. When the post office was established it was called Collamer for the postmaster general. The name was afterward changed to Merrimac. It has been reported the Native American word Merrimack means "sturgeon" or "swift water."

Magnus Swenson constructed the dam at Prairie du Sac which, at the time, was alleged to be grander than any other west of Niagara Falls. The completion of the dam on September 10, 1914, changed the Wisconsin River through the Merrimac community and Lake Wisconsin was born. The lake’s diverse community of year-round homes, seasonal cottages, and new fishing spots changed this once purely agricultural community into a year-round destination for residents and tourists alike.

Sources:
Wisconsin Historical Society
Fox Wisconsin Heritage Parkway