He later served as United States Consul at Ningbo, China and was also judge of the "mixed court" at Shanghai.

In June 1843, while engaged in the practice of medicine in Philadelphia, he received a message from the Board of Foreign Missions of the Church that he was needed to go to China as a pioneer and medical missionary.

In 1868 or 1869, the McCartees adopted Kin Yamei (1864–1934), the daughter of Christian colleagues who died from disease when she was two years old.

In addition to his medical work, he became an adviser and interpreter for American officials and was later vice-consul in Chefoo (present-day Yantai) and Shanghai.

In this capacity in May 1861, at the request of United States Flag-Officer Stribling, he entered Nanjing, across the battle lines and helped persuade the "Heavenly King", Hong Xiuquan of the Taipings to promise "non-molestation not only to Americans and Christians, but to all Chinese in their employ."

By this effort large numbers of native Christians and their friends were rescued when the Taiping army entered Ningbo.

In 1872, when the coolies of the Peruvian ship Maria Luz were freed by the Japanese government upon his suggestion, a commission was appointed from Beijing to proceed to Tokyo to bring home the freed men, and McCartee was nominated secretary and interpreter, receiving for his services a gold medal and complimentary letters.

In 1879, he advised Ulysses S. Grant, the former U.S. president, mediating on the Ryukyu Islands, although both China and Japan rejected his compromise.