although mifflin's family had been quakers for four generations, he was expelled from the religious society of friends when he joined the continental army, because his involvement with the military contradicted that faith's pacifistic doctrines. mifflin became a member of the american philosophical society in 1768 and served for two years as its secretary. he served from 1773 to 1791 as a trustee of the college and academy of philadelphia (now the university of pennsylvania), including two years as treasurer (1773–1775).