Diekema was born in Holland, Michigan, where he attended the common schools and graduated from Hope College in 1881.

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.

He was a delegate to the 1896 Republican National Convention and a member of the Spanish Treaty Claims Commission from 1901 until he resigned in May 1907.

He was elected April 27, 1907, as a Republican from Michigan's 5th congressional district to the Sixtieth Congress, to fill the vacancy caused by the resignation of William Alden Smith.

Diekema was appointed United States Minister to the Netherlands by President Herbert Hoover on August 20, 1929, and served until December 1930.