Diehl grew up in Fort Worth, Texas, and is of Luxembourgian descent.

[1] At the age of 15, he moved with his mother to Mission Viejo, California for training purposes and he completed his schooling at Mission Viejo High.

[2] In 1990, Diehl earned a place on the U.S. team for the Goodwill Games after winning the 1,000 m freestyle at the short course national championships.

He came eighth in the 800 m freestyle at the Goodwill Games, held in Seattle.

[4] While attending Stanford University, Diehl competed in varsity swimming for the Cardinal.