 On August 7, 2006, Mock was acquired by the Nationals along with fellow minor league pitcher Matt Chico in a trade for veteran right-handed pitcher Liván Hernández.After joining the Nationals organization, he pitched for the Harrisburg Senators of the Double-A Eastern League, compiling a 10.26 ERA in his first 4 starts, all losses.His 2006 season ended September 2, after undergoing surgery to repair a tear in the patella tendon of his knee.