After finishing his Ph.D., Willard returned to Michigan to rejoin the faculty; he became a full professor in 1922 and retired from the university, assuming professor emeritus status, in 1951. He was designated the Henry Russel Lecturer in 1948, noted as the university's highest distinction. He was known for his strong teaching skills and continued teaching at a variety of institutions after his retirement.
During his tenure at Michigan, Willard wrote several widely used and positively reviewed chemistry textbooks and laboratory course manuals, often with former students as coauthors. He also performed consulting work for local industry throughout his career, serving as the Director of the Chemistry and Metallurgy Laboratories for Detroit's Bureau of Aircraft Production in 1917-18 and as a long-term consultant for the Parker Rust-Proof Company.
Willard served as a director of the American Chemical Society from 1934 to 1940 and received the ACS' Fisher Award in Analytical Chemistry in 1951. He was the inaugural recipient of the Anachem Award given by the Association of Analytical Chemists in 1953.