He continued to play the next two years as the third-string catcher before serving as Dave Duncan's backup in 1971. Tenace entered the 1972 season backing up Duncan, but was given a chance to show his abilities by being made the team's regular catcher in the post-season. Tenace took full advantage of this opportunity, excelling in the 1972 playoffs and World Series. In the 1972 American League Championship Series against the Detroit Tigers, he drove in the clinching run in Oakland's 2–1 victory in game 5; it was his only hit of the series.
He put himself in the spotlight once again in Game 1 of the 1972 World Series when the Athletics faced the Cincinnati Reds. He became the first player to hit home runs in his first two World Series at bats and drove in all three runs in the A's 3–2 victory. In Game 4, the A's were losing 2–1, with their only run to that point coming on Tenace's solo home run. A ninth-inning one-out rally consisting of four singles, with Tenace's the second and the rest coming from pinch hitters, won the game against Cincinnati's ace relief pitcher, Clay Carroll. He hit a three-run home run in the A's Game 5 loss. Before Game 6 of the 1972 Series, he was the target of a death threat. In Game 7, he was once again the hero, driving in two runs in a 3–2 victory for Oakland. In total, he went 8 for 23 in the Series, with 4 home runs and nine RBI to earn the World Series Most Valuable Player Award.
Tenace's heroics helped him earn a full-time job in Oakland's lineup. He served as the team's starting first baseman for two seasons, while still serving as the backup catcher to Ray Fosse. He had his roles reversed in 1975, starting at catcher while backing up first base. As a regular starter for the A's, Tenace had a low batting average but a fair amount of power, hitting 20 home runs in four consecutive years in Oakland, finishing among the top 10 home run hitters in the American League each year. He further made up for his lack of a high batting average by sporting a tremendous batting eye. He drew over 100 walks in a season three times for Oakland, and led the American League in walks in 1974, making up for a career-low .211 average that year. Statistically, his best year with Oakland was in 1975, when he hit a career-best 29 home runs and drove in 83 runs, drew 106 walks, finished 18th in the American League Most Valuable Player Award balloting and was selected to be the starting first baseman for the American League in the 1975 All-Star Game.
Also while a member of the A's, Tenace hit what turned out to be the final home run in the history of Kansas City's Municipal Stadium on September 30, 1972, in a 10-5 Oakland victory over the Kansas City Royals.
Tenace was one of several Athletics who became free agents after the 1976 season and participated in a newly created re-entry draft, in which teams acquired the rights to negotiate with veteran free agents. Tenace and teammate Rollie Fingers were the first players from that draft to sign, with both joining the San Diego Padres in December of that year. In four years as a starter with the Padres, his power numbers dropped in part due to the cavernous dimensions of San Diego Jack Murphy Stadium, only reaching 20 home runs once; but his batting eye remained, recording three more seasons of 100 walks, with a career best of 125 in 1977. In 1979, Tenace led National League catchers with a .998 fielding percentage, committing only 1 error in 94 games. He recorded an on-base percentage of over .390 in each of his years in San Diego, and finished 3rd in the National League in that department in 3 consecutive years.
Tenace advocates a more aggressive approach to hitting. Under his guidance, hitters spend less time working the count and more time preparing to hit. He stresses the mental part of hitting, such as the mental preparation for what a pitcher will do, rather than just the physical aspect. Under Tenace, the philosophy of hitting can be described as "Grip It And Rip It" and more of an old school approach to hitting.
Before Gaston was hired the Blue Jays had a record of 35–39 and since Gaston and his coaches took over the Blue Jays went 51–37 to finish with an 86–76 record on the season. The offensive improvements under Gaston are one of the reasons for the resurgence and as the hitting coach, Tenace has been credited with rejuvenating a stagnant offense. Adam Lind credited his great offensive numbers in the month of July and August to the coaching styles of Gaston and Tenace, saying: "The thing is, a lot of people can teach you how to hit, but not a lot of people can teach you how to hit in the big leagues," referring to the major league experience of Gaston and Tenace. Blue Jays' center fielder Vernon Wells has said he likes the approach to hitting that Tenace teaches: "One thing Gene talks about is having that approach mentally when you go up there, in your mind you already guarantee yourself that one run on third, but you want to do more than that. Gene always says that first runner is a gimme, you should be able to come through in pretty much every situation like that. And then you try to do more damage than just that one run".
In a 15-year major league career, Tenace played in 1,555 games, accumulating 1,060 hits in 4,390 at bats for a .241 career batting average along with 201 home runs, 674 runs batted in and an on-base percentage of .388. He not only caught nearly 900 games, but also played first base over 600 times. Tenace ended his career with a .986 fielding percentage as a catcher and a .993 fielding percentage as a first baseman. He reached 20 home runs in five of his seven seasons as a regular, with a high of 29 in 1975. After becoming an everyday player in 1973, he did not have an on-base average below .370 until his final year; his OBP was above .400 five times and over .390 (about 60 points above the league average) an additional three times, ending his career with an impressive .388 on-base percentage. Six times he drew more than 100 bases on balls, and he led his league twice. He set the American League record for having the lowest batting average while leading the league in walks in 1974 when he posted a .211 batting average with a league-leading 110 walks. In 1977, he had a .415 on-base percentage while posting a .233 batting average, the second lowest batting average with a .400 on-base percentage in major league history. Less than half of his career trips to first base came via base hits, reaching 1,075 times through walks (984) and being hit by pitches (91) as opposed to only 1,060 hits.