Ordained in 1794, he accepted a position at the Congregational Church in Milton where he stayed until 1804 when poor health obliged him to beg relief from his pastoral duties.
Two years later on October 31, 1809, McKean did accept the appointment as the second Boylston Professorship of Rhetorick and Oratory when the first Boylston Professor, John Quincy Adams, resigned in order to become the federal government's minister to Russia[5] Milton Academy was established by an act of the Massachusetts Legislature on March 3, 1798, "...for the purpose of promoting piety, religion & morality & for the education of youth in such Languages, & in such of the liberal arts & sciences, as the Trustees of the said Academy shall direct..."[6]: 476 As part of this legislation, McKean was appointed a Trustee of Milton Academy along with Fisher Ames, William Aspinwall, Samuel Bass, Nathaniel Emmons, Thadeus Mason Harris, Zachariah Howard, George Morey, Eliphalet Porter, Thomas Thatcher, Stephen Metcalf, John Read, Edward Robbins, and Ebenezer Thayer.
93 Court Street, next door to Barditt's Book-Store.” The catalog of the auction [7] runs to 51 pages and lists 1,541 titles among which are a number in mathematics, chemistry, astronomy, and archaeology.
[8] The 266-page book organizes the approximately 3,500 titles in the collection into fifteen classes including Theology, Law, Mathematics and Natural Philosophy, Chemistry and Mineralogy, and Poetry and Dramatic Works.
[9]: 67 In the telling of a tract [10] by Athenæum staff member, James Belliveau, it was McKean who added a hand-written letter K to the title page of a large number of the books listed in the 1810 catalog.