The A's announced Friday that Williams will join manager Bob Melvin's staff. Melvin was a coach in Arizona for two seasons late in Williams' career with the Diamondbacks.
Williams played 17 years in the majors, beginning with the cross-bay Giants. He also played for Cleveland and Arizona, finishing his career with 378 homers, five All-Star appearances and four Gold Glove Awards at third base.
Williams then spent five years as a coach with the Diamondbacks before managing Washington for two seasons in 2014-15. He had a 179-145 record and won the NL Manager of the Year award his first season.