After the death of her father in 1762, she and her mother joined her brother in Providence, R.I. where he had established a printing shop, and where both mother and daughter began their careers as printers. Mary Katherine actively worked in publishing the weekly Providence Gazette until the end of 1768 when she joined her brother’s printing office in Philadelphia, where he published the Pennsylvania Chronicle. Though the publication remained under the brother’s name, William Goddard, Mary Katherine managed the shop, one of the largest in the colonies. In May 1773, William started a new printing business in Baltimore and began Baltimore’s first newspaper, the Maryland Journal. In February 1774, the Philadelphia shop closed and Mary Katherine moved to Baltimore to take over the new plant and newspaper.

The May 10, 1775 issue of the Maryland Journal made official what had been in practice for over a year when the colophon was changed to read, “Published by M. K. Goddard.” Mary Katherine proved to be a steady, impersonal newspaper editor and during the Revolution she was usually Baltimore’s only printer. From her press, in January 1777, came the first printed copy of the Declaration of Independence to include the names of the signers. Mary Katherine Goddard was also responsible for issuing several Almanacs, while in Baltimore, which now hold a place in the Maryland Historical Society.

In 1775, Mary Katherine became postmaster of Baltimore, probably the first woman so appointed in the colonies, and certainly the only one to hold so important a post after the Declaration of Independence. She continued in the office for fourteen years until in October 1789 when, much against her will, she was relieved on the ground that someone was needed who could visit and superintend the Southern department of the postal system. The authorities believed that this responsibility involved more traveling than a woman could manage. The esteem in which Goddard was held is revealed by the fact that over two hundred of the leading businessmen of Baltimore endorsed her petition to the Postmaster General to retain her position. Remaining in Baltimore, she continued to operate, until 1809 or 1810, the bookshop she had begun as an adjunct of the printing business. She died in Baltimore at the age of seventy-eight and was buried in the graveyard of the St. Paul’s Parish.

Because Mary Katherine did not engage in public controversies but remained an impersonal editor, there are few statements that reflect her personal point of view. Her brother described her as, “an expert and correct compositor of types,” and respect for her abilities as a postmaster is shown in letters by such diverse people as Ebenezer Hazard and Thomas Jefferson.