 In 1883, he graduated from the law department of the University of Michigan at Ann Arbor, was admitted to the bar, and commenced practice in Holland.Diekema became a city attorney and a member of the Michigan State House of Representatives serving from 1885 to 1891 from Ottawa County 1st District, serving as speaker from 1889 to 1890.He became mayor of Holland in 1895 and chairman of the Michigan Republican Party ten consecutive years from 1900 to 1910.