Educated in schools and by private instruction. (Doctor of Letters, Tufts, 1905). Studied architecture at Salem, and Boston.
He was appointed special agent for the Massachusetts Bureau of Statistics of Labor in 1879 and upon becoming its chief in 1888 abandoned his professional practice. He was also the chief supervisor of the 1890 and 1900 censuses for Massachusetts. He served as the chief of the Bureau of Statistics until 1906.
In 1906, he retired from the Bureau of Statistics to become the librarian of the Boston Public Library.
He announced his resignation as the librarian of the Boston Public Library in December 1916. Wadlin died in November 1925 after an operation for kidney trouble.
He was a member of the Massachusetts legislature in 1884-1888, was supervisor of the United States census in 1890 and in 1900 and also of the Massachusetts census of 1895. Wadlin was a member of the American Economic Association, the American Social Science Association, and the American Statistical Association, serving as vice president of the latter organization.
Married Ella Frances Butterfield, September 8, 1875.