Palmeiro was recruited by Ron Polk and enrolled at Mississippi State University, where he played college baseball for the Bulldogs in the Southeastern Conference (SEC). He was the first of two players to have ever won the SEC triple crown, both of whom are Mississippi State products. A teammate of Will Clark, the two were known as “Thunder and Lightning”. Clark and Palmeiro were known to dislike each other, dating back to their time at Mississippi State. On June 11, 1985, Palmeiro signed with the Chicago Cubs as the 22nd pick in the 1st round of the 1985 draft.

Palmeiro debuted on September 8, 1986 in a game between the Chicago Cubs and Philadelphia Phillies at Wrigley Field, as a left fielder. In his first season, he played 22 games, having a .247 batting average while having 12 RBIs and 3 home runs. In the next season, he played in 84 games, garnering 61 hits, 14 home runs and 30 RBIs with a .276 average. During his tenure with the Cubs, he normally played left field, though occasionally he would play other outfield positions or first base. Palmeiro was the runner up to National League batting champion Tony Gwynn in 1988 with a .307 batting average, only six points below Gwynn’s. He had 178 hits, 8 home runs and 53 RBIs in 629 plate appearances and 152 games. He was named to the 1988 MLB All-Star Game, the first in his career.

Upon moving to the American League, Palmeiro was primarily used as a first baseman or designated hitter. Palmeiro blossomed as a hitter while with the Rangers, leading the league in hits in 1990 and doubles in 1991. In 1989, he hit for .275 that season, having 64 RBIs, 8 home runs and 154 hits. In 1990, he was third in the American League in batting. He hit for .319 during the season, while having 89 RBIs and 14 home runs and leading the league in hits. He hit 49 doubles in 1991 (a season high along with career high), while having 203 hits, 26 home runs and 88 RBIs for a .322 batting average. He was named to his second career All-Star Game that year. He dipped in every category in 1992, having 163 hits, 22 home runs, 85 RBIs and a .268 batting average.

Starting in 1995, Palmeiro began a streak of 38+ home run years that continued through the 2003 season. He hit 373 home runs during this nine-season span, while also driving in over 100 runs in each of these seasons. However, Palmeiro never led the league in home runs, and he is history’s most prolific home run hitter to have never won the home run crown.

That season, he led the team in home runs, batting average, and runs batted in. In the 1996 season, he hit 39 home runs, 181 hits, and 142 runs batted in and helped the Orioles qualify for the American League Wild Card. This was both his first postseason appearance in his career along with the first postseason appearance for the Orioles since 1983. In his first postseason, he had seven total hits, six RBIs and a combined .205 average while playing in all nine games the Orioles played, as they beat the Cleveland Indians in the 1996 American League Division Series to advance to the 1996 American League Championship Series before being beat by the New York Yankees. He finished 6th in MVP balloting, his highest finish up to that point in his career.

Despite having 109 strikeouts (a career high) and a .254 batting average, he had 110 RBIs, 156 hits along with 38 home runs in 1997 as he helped the team win their division for the first time since 1983. He also was awarded the Rawlings Gold Glove Award that season, his first ever. In the 1997 American League Division Series, the Orioles beat the Seattle Mariners in four games. In the postseason, he had 10 combined hits along with 2 RBIs and 1 home run and a .265 average in the 10 games he played in the postseason, as the Orioles once again advanced to the ALCS after beating the Seattle Mariners before losing to the Indians in six games. This would be the last time the Orioles would make the playoffs until 2012 and last time they would win a division title until 2014. The next season, his last with his first stint with the Orioles, he hit 43 home runs, to lead the team for the 4th time. He had 183 hits along with 121 RBIs and a .296 batting average. He was named to the 1998 MLB All-Star Game, his first since 1991. He also won a Gold Glove for the second straight year along with his first Silver Slugger Award, while finishing 18th in MVP balloting. In his time with the Orioles, he received MVP votes each year, though he never finished higher than 6th.

Palmeiro was offered a 5-year, $50,000,000 deal to stay with the Orioles, but instead agreed to a 5-year, $45,000,000 contract to return to the Rangers in 1999, citing a desire to be close to family (he had remained in the Dallas area during his time with the Orioles).

Palmeiro played an average of 157 games per season in his second tenure with the Rangers. He had 47 home runs in his first season back with the Rangers, while hitting for .324 and having 183 hits. He finished 5th in the MVP balloting, his highest finish ever while being named to the 1999 MLB All-Star Game, his fourth and final selection. He was awarded the Gold Glove for the third straight year along with winning the Silver Slugger Award for his second straight and final year. The Gold Glove win was not without controversy, as he appeared in only 28 games as a first baseman while playing 128 games as the designated hitter. He finished 5th in the MVP balloting, having garnered a .324 batting average, 47 home runs, and 148 RBIs, all improvements from the previous season. He hit over 100 RBIs in each of his five seasons, along with 805 hits. He hit 214 home runs with the team in that span, though his batting average dipped each year, hitting .260 in his final year in 2003. His first season resulted in a division title. In the 1999 American League Division Series, his team was swept by the Yankees in three games. He had three hits and a .273 average for the series. This was his last postseason appearance. The team would finish last place in his remaining four seasons. Palmeiro was one of the few bright spots for the Rangers from 2000–2003, when he left the Rangers to re-join the Orioles.

On May 11, 2003, Palmeiro hit his 500th home run off David Elder in a game against the Cleveland Indians.

Palmeiro re-signed with the Baltimore Orioles in 2004, on a 1-year, $4,000,000 contract. In his press conference, he claimed he was “a little bit older, a little bit wiser”. He also said he didn’t want to retire until he rejoined the Orioles and if he was inducted to the Hall of Fame, he would do so as an Oriole. His power fell significantly his first season back with the Orioles, having 142 hits, along with 23 home runs, 88 RBIs and a .258 batting average, all drops from the previous season. Despite this, he became one of only six players (the other five being Ken Griffey Jr, Mark McGwire, Jimmie Foxx, Albert Pujols and Manny Ramírez) in history to hit at least 200 home runs for two different clubs, doing so on September 13, 2004 off Justin Miller. Entering 2005, he was just 78 hits shy of 3,000. He hit his 563rd home run that season, passing Reggie Jackson on the all-time list. Just a few weeks later, on July 15, Palmeiro joined Hank Aaron, Willie Mays, Alex Rodriguez, and Eddie Murray as the only players in major league history to get 3,000 hits and 500 home runs. Not long after his achievement, Palmeiro was suspended 10 days for testing positive for steroids. He claimed he received a tainted vitamin b12 shot from Miguel Tejada. Upon returning, he only played seven more games. He was booed by the home fans. He was sent home due to a nagging injury, but was told to stay home – largely because of his attempt to implicate his teammate. As of April 2016, he has yet to return to Baltimore. In his final season, he had 98 hits, 18 home runs, 60 RBIs and a .266 batting average in 110 total games played. He finished his career with 2,831 games played, the most by any player who never played in the World Series.

Former Rangers teammate José Canseco identified Palmeiro as a fellow steroid user in his 2005 book, Juiced: Wild Times, Rampant ‘Roids, Smash Hits & How Baseball Got Big, and claimed he personally injected Palmeiro with steroids. On March 17, 2005, Palmeiro appeared at a Congressional hearing about steroids in baseball and, while under oath, denied ever using steroids and stated, “Let me start by telling you this: I have never used steroids, period. I don’t know how to say it any more clearly than that. Never.”

On August 1, 2005, Palmeiro was suspended for ten days after testing positive for a steroid. The Washington Post reported that the steroid detected in Palmeiro’s system was a “serious” one. According to The New York Times, Palmeiro tested positive for the potent anabolic steroid stanozolol. In a public statement, Palmeiro disclosed that an appeal of the suspension had already been denied. He released a statement saying, “I have never intentionally used steroids. Never. Ever. Period. Ultimately, although I never intentionally put a banned substance into my body, the independent arbitrator ruled that I had to be suspended under the terms of the program.” According to Palmeiro, all of his previous tests over the two years including the 2003 sealed test were negative, and a test he took just three weeks after his positive test was also negative. The House Government Reform Committee would not seek perjury charges against Palmeiro, although they were not clearing him.

Palmeiro returned to Camden Yards following his 10-day suspension on August 11, 2005, although he did not play in the lineup until August 14. Coincidentally, this was the date that had been planned as “Rafael Palmeiro Appreciation Day” in celebration of his 500-home run, 3,000-hit milestone. It was canceled after Palmeiro’s suspension. The Baltimore Sun reported that Palmeiro never offered an explanation for his positive test to the MLB arbitration panel, which ran contrary to his public statements. ESPN later reported that Palmeiro implicated Miguel Tejada to baseball’s arbitration panel, suggesting a supplement provided to him by Tejada was responsible for his positive test. This supplement was supposedly vitamin B12, though it could have been tainted. Tejada and two unnamed teammates provided B12 samples to the panel, which did not contain stanozolol. However, the committee did say they found “substantial inconsistencies between Mr. Tejada’s accounts and the accounts of players A and B.” Tejada, who said he received shipments of B12 from the Dominican Republic, was later implicated for steroid use in the Mitchell Report.

Palmeiro continues to strongly deny ever having used steroids intentionally, telling The Baltimore Sun in June 2006, “Yes sir, that’s what happened. It’s not a story; it’s the reality of what happened”, and “I said what I said before Congress because I meant every word of it.” Palmeiro passed a polygraph test in which he was not asked if he ever used steroids, but in which he did state that he unknowingly ingested them via a B12 injection. A 2005 New York Times article expressed one writer’s belief that Palmeiro’s story could perhaps be the truth.

In December 2007, Palmeiro was included in the Mitchell Report in which it was alleged that he used performance-enhancing drugs during his career. The report did not provide any new evidence and only recapped allegations made by José Canseco, Palmeiro’s appearance before Congress, and his subsequent failed drug test. The report also details a conversation Larry Bigbie alleges he had with Palmeiro where he claims “Palmeiro asked him about his source of steroids and human growth hormone (the source was Kirk Radomski) and how the substances made him feel.” Bigbie also stated that “Palmeiro denied in those conversations that he had ever used performance-enhancing substances himself.”

On December 20, 2007, Palmeiro was also named in Jason Grimsley’s unsealed affidavit as a user of amphetamines prior to their being banned by MLB.

Palmeiro was inducted into the Mississippi State University Hall of Fame on October 11, 2008. In 2009, he was inducted into the National College Baseball Hall of Fame. In 2012 Rafael Palmeiro was inducted into the Mississippi Sports Hall of Fame.

Palmeiro became eligible for induction into the Baseball Hall of Fame in 2011. He received 64 votes, or 11% of total ballots cast; the threshold for entry is 75%. According to Barry M. Bloom on Major League Baseball’s official website: “Palmeiro should have been a sure-fire first-ballot inductee, as a member of the 500-homer, 3,000-hit club, but was suspended in 2005 after testing positive for steroid use. He received only 11 percent of the vote.” On January 8, 2014, Palmeiro received insufficient support to remain on the Hall of Fame ballot in future years, when he appeared on only 4.4% of ballots.

On September 17, 2015, it was announced that Palmeiro would sign with the Sugar Land Skeeters of the Atlantic League of Professional Baseball to play alongside his son, Patrick Palmeiro, for one game. Palmeiro appeared in only one game for the Skeeters on September 18, 2015. Skeeter fans quickly gave him the nickname “The Saccharin Slugger.”

Palmeiro lives in Colleyville, Texas, with his wife. His son Preston played baseball for North Carolina State University and was drafted by the Baltimore Orioles organization in 2016.

Palmeiro announced on January 5, 2018 that he wanted to return to Major League Baseball. On May 9, 2018, it was announced that Palmeiro and his son Patrick signed with the Cleburne Railroaders of the American Association of Independent Professional Baseball. On May 22, 2018 he hit his first professional home run since July 2005 when he was with the Orioles. He finished the season with a .301 batting average, 6 home runs, and 22 RBIs despite being limited to only 31 games due to injuries. On March 11, 2019, the Railroaders announced that Palmeiro and his son would return for the 2019 season. However, on May 15, the Railroaders released the pair, stating that Rafael wasn’t fully healthy and wouldn’t be able to play a full season after undergoing knee surgery earlier in the year.