John had a quick path to the majors, debuting with the Senators less than two years after signing his first pro contract. In his first at-bat, on September 5, 1962, he hit a solo home run. He only spent a few months with Washington the following year, but manager Gil Hodges penciled him into the lineup as a regular for the only time in his career during the 1964 campaign. In a career-high 482 at-bats (he didn't top 300 in any other season), Kennedy hit .230 with 27 extra-base hits and 35 RBI. He played mostly at third base, but saw a good amount of time at shortstop. He was involved in the trade that brought Frank Howard and Ken McMullen to D.C., and would play in back-to-back World Series in his two seasons in Los Angeles despite peaking at .201 as a Dodger hitter. John spent 1967 in Yankee pinstripes but failed to remind fans of Phil Rizzuto, checking in at .196.
Midway through the following season, the ex-Pilots (now the Brewers) dealt John to Boston, where he played some of his best ball. In back-to-back seasons (1970-1971), he posted his two highest batting and slugging averages (.255/.400 and .276/.412). All told, he spent four-plus seasons in a Red Sox uniform before retiring in 1974 at age 33. Kennedy managed in the Red Sox and Athletics organizations for four years, and has also coached and scouted. More recently, he managed the North Shore Spirit of the independent Northeast League and Canadian-American Association, piloting the club to three postseason appearances in four years.
Fun fact: John owned pitcher Stan Bahnsen, batting 9-for-20 off of him, with an OPS (on-base-plus-slugging percentage) of 1.100. He also had great numbers against Dick Drago, Dean Chance, and Mickey Lolich; he homered twice against both Chance and Lolich.