Throughout his career, Johnson maintained a personal and political friendship with George Washington,[2] who gave him a recess appointment as an associate justice of the Supreme Court in August 1791.

He also served as a senior brigadier general in the Maryland militia from January 1776 to February 1777,[5] commanding troops sent to aid Washington during his retreat through New Jersey in the winter.

[7] Their former factory, Catoctin Furnace, is now part of a state park near Camp David, just north of Frederick, Maryland.

Although Johnson was not a commissioner,[8] the resulting conference agreed to regulate and settle the jurisdiction and navigation on their mutual border of the Potomac River.

[10] In September 1789, President George Washington nominated Johnson to be the first federal judge for the District of Maryland, but he declined the appointment.

In January 1791, President Washington appointed Johnson, with David Stuart and Daniel Carroll, to the commission that would lay out the federal capital in accordance with the Residence Act of 1790.

[11] On August 5, 1791, Johnson received a recess appointment from Washington as an associate justice of the United States Supreme Court, to the seat vacated by John Rutledge,[12] and was sworn into office on September 19, 1791.

[2] Johnson suffered very poor health for many years, and cited it in declining Washington's 1795 offer to nominate him for Secretary of State, as Thomas Jefferson had recommended.

On February 28, 1801, President John Adams named Johnson chief judge for the District of Columbia; he was confirmed for the post, but declined the appointment.

[12] His daughter Ann had married John Colin Grahame in 1788, and in his later years Johnson lived with them in a home they had built in Frederick, Maryland.

[13][14] Johnson was one of the first investors in the Illinois-Wabash Company, which acquired a vast swath of land in Illinois directly from several Indian tribes.