A small, skinny player[dubious– discuss] at 5 ft 10 in (1.78 m) and 160 pounds (73 kg), the Cuban-born Campaneris was a key figure on the A's of the 1960s and 1970s. In his debut with Kansas City on July 23, 1964, Campaneris hit two home runs, the first coming on the first pitch thrown to him by Jim Kaat. He is one of five players in major league history to hit two home runs in his first game; Bob Nieman (1951), Mark Quinn (1999), J.P. Arencibia (2010), and Yasmani Grandal (2012) are the only others to accomplish this feat.
In 1965, Campaneris led Kansas City in batting average (.270), and led the league in triples (12) and stolen bases (51), the latter mark being the highest total by an Athletic since Eddie Collins's 58 in 1914 with what were then the Philadelphia Athletics and breaking Luis Aparicio's nine-year run of the American League stolen base title. In 1966 he finished 10th in the voting for the AL's Most Valuable Player award after having a similar season at the plate, but playing more regularly at shortstop.
When the A's moved to Oakland in 1968, he had perhaps his finest year, leading the league in hits (177), steals (62), and at bats (642); the last mark was an Oakland record until Johnny Damon broke it in 2001. Campaneris enjoyed another fine year in 1970, batting .279 with career highs of 22 home runs and 64 runs batted in while leading the league in steals for the fifth time (42) and scoring 97 runs. He improved consistently on defense; his six double plays in an extra-inning game on September 13 of that year set an AL record, and in 1972 he led AL shortstops with 795 total chances while also breaking Collins' franchise record of 376 steals. An avid bunter, he led the league in sacrifice hits in 1972 (20), 1977 (40) and 1978 (25).
In his postseason career he had 11 RBI with 3 home runs, 4 doubles and a triple, scored 15 runs, and stole 6 bases in 37 games.
In Game 2 of the 1972 American League Championship Series against the Detroit Tigers, Campaneris already had three hits, two steals and two runs when in the 7th inning he faced pitcher Lerrin LaGrow. After a pitch hit him in the ankle, Campaneris threw his bat at LaGrow. The benches cleared, with Detroit manager Billy Martin going after Campaneris; both Campaneris and LaGrow were ejected. Campaneris was fined and suspended for the rest of the ALCS, as well as the first seven games of the next season, but was permitted to play in the World Series win over the Cincinnati Reds.
In his 19-year career Campaneris batted .259 with 79 home runs, 646 RBI, 1181 runs, 2249 hits, 313 doubles, and 86 triples in 2328 games. His 649 stolen bases placed him 7th in major league history, and behind only Ty Cobb and Eddie Collins in the AL. His Athletics record of 566 steals was broken by Rickey Henderson in 1990; Henderson also surpassed his Oakland records for career triples and at bats. Campaneris retains the Athletics franchise records for career games (1702), putouts (2932), assists (5021) and double plays (934) at shortstop.
Campaneris lives in Scottsdale, Arizona, and often participates in Old-Timers' games around the country. He also conducts baseball camps and is an active participant in the Major League Baseball Players Alumni Association, often playing in charity golf tournaments.