Born in Crawfordsville, Indiana, Dietz was signed by the San Francisco Giants as an amateur free agent out of Greenville, South Carolina before the start of the 1960 season.

[4] In February 1968, the Giants were in need of good infielders, and with four young catching prospects in their system, including Dietz, club president Chub Feeney decided to trade Haller along with a player to be named later, to the Los Angeles Dodgers for Ron Hunt and Nate Oliver.

[12] Dietz was an integral member of the 1971 Giants team that clinched the National League Western Division title.

[6] The Giants had a nine-game lead over the Los Angeles Dodgers on August 1, but held on during a pennant race to win the division by a narrow one-game margin.

[1][6] In the only post-season appearance of his career, Dietz had only one hit in 15 at-bats, as the Giants lost to the eventual world champion Pittsburgh Pirates in the 1971 National League Championship Series.

[14] In 1972, Dietz fell from favor with the Giants' management for his role as player representative during the 1972 Major League Baseball strike that delayed the start of the season.

Other reports stated that the Giants gave up on Dietz because of his defensive liabilities, having led National League catchers in passed balls in 1970 and 1971.

[1] Among major league catchers, he ranks third overall in career on-base percentage behind only Mickey Cochrane and Wally Schang.