From 1853 to 1856, Key was a draughtsman and map maker for the United States Coast Survey in Washington, District of Columbia. In 1863, Key was commissioned as second lieutenant in the Corps of Engineers at Charleston, where he recorded the federal siege in his paintings. From 1870 to 1873, Key had a studio in San Francisco. In May 1871 his work was part of the first exhibition by the San Francisco Art Association.
After spending two years studying in Europe, Key returned to the United States of America and set up studios in Boston and New New York
The next year one hundred of his paintings were on display in the Boston Athenaeum.
In 1869, Key moved to the East Coast and became a member of the Society of Washington Artists and the Boston Art Club.