After attending the Westtown School from 1892 to 1897, he obtained his Bsc in Mechanical Engineering from the University of Pennsylvania in 1901.
He volunteered to lead Quaker relief efforts in Germany after World War I, and was a founding member in the 1920s of the Business Problems Group of Philadelphia Yearly Meeting.
[7] In 1938 Yarnall traveled with George Walton and Rufus Jones on a mission to Nazi Germany to try to help Jewish people there after the Kristallnacht.
[8] In 1941 Yarnall was awarded the Hoover Medal, and in 1942 he received the honorary Doctor Engineering from Lehigh University.
He was elected president of the American Society of Mechanical Engineers for the year 1946–47, and in 1947 received the honorary Doctor of Science from Haverford College.