He has starred in both leagues, going from a young, middle-of-the-order bat to a veteran pinch-hitter who will turn 42 years old before the season is complete.
For much of his career, Thome has been accustomed to receiving four at-bats a game. At this stage, however, those at-bats are not guaranteed with the Phillies.
"My role is what my role is and, that's come to the ballpark every day," Thome said. "If you're not playing that day, try to be a leader on the bench, watch the pitchers, maybe give a little input. But also, get myself ready for late in the game to come off the bench and get a big hit."
Thome made the most of Philadelphia's recent nine-game Interleague road trip. He was used as the team's designated hitter in each contest, and produced in a big way.
The slugger batted .333 (12-for-36), with four homers and 14 RBIs in a road trip that stopped in Baltimore, Minnesota and Toronto. Outside of the at-bats he got at extended spring camp and on a Minor League rehab assignment, Thome had just two plate appearances since coming off the disabled list. He had missed over a month with a lower back strain.
But don't expect to see him in the lineup when the Phillies resume play Tuesday in Philadelphia.
"It will be kind of tough," Phillies manager Charlie Manuel said about Thome going back to the bench. "But at the same time, we have to keep him ready to pinch-hit.
"First base is out of the question."
Thome's lower back problems, which he says have plagued him for the last seven to 10 years, prevent him from playing the field. This takes a valuable bat out of a lineup that could really use one.
Injuries have ravaged the Phillies this season. It will surely hurt not being able to use one of their hottest hitters outside of late pinch-hitting opportunities. It's a role that will also almost assuredly disrupt any sort of rhythm Thome has established at the dish.
"The main thing for me has always been getting a feel, getting at-bats," Thome said, when asked how he stays sharp. "Let's face it, pinch-hitting is a challenge. You get one at-bat and you're facing seventh-, eighth-, ninth-inning guys, and they have a job to do. I don't want to say it can't be done, but I also think it's something you need to bear down on."
His role may have changed from the days when he hit 40-plus home runs in four consecutive years from 2001-04, including a career-high 52 in 2002 with the Indians. Despite this, no one in the Phillies clubhouse doubts what he has left.
"There are no substitutes for at-bats," Phillies hitting coach Greg Gross said. "Jim has always been used to being an everyday player. So, the adjustments to only getting a few at-bats a week is tough.
"But Jim can still swing the bat. It's more about how much his back can withstand what he does."
Although Thome's back has limited him at times for much of the past decade, he continues to cement his legacy among the greats.
During his latest stop in Toronto, Thome joined Alex Rodriguez, Reggie Jackson and Darrell Evans as the only players in Major League history to hit at least 100 home runs for three different teams.
"He's one of the best power hitters of this generation, there is no doubt about that," Gross said. "He ranks right there with all the other great ones."
The next homer Thome hits will tie him with Sammy Sosa for seventh on the all-time list. The next walk he draws will tie him with Mickey Mantle for the seventh most in Major League history.
"As a player, you don't come into baseball and ever dream or think that you're going to do or accomplish those things," said Thome, whose home run Sunday was the 608th of his career. "You do feel proud. You've played a long time, worked your rear end off and tried to do things the right way. Baseball is a roller-coaster ride. The back of a baseball card isn't always going to be the same. I have been fortunate. I had an Eddie Murray and a Dave Winfield and guys that I really looked up to and thought, 'What made them great?' It's their longevity.
"Being able to play into my 40s is pretty awesome."
Thome is one of only three players alongside Babe Ruth and Barry Bonds who have hit more than 600 career home runs and walked more than 1,700 times. He is also only one of a handful of active players with a career on-base percentage above .400.
He's accomplished so much, yet what he desires most has eluded him his entire career.
"Win a World Series," Thome said, when asked what more he wants out of the game. "I think that should be everybody's dream. You can do a lot of great individual things but, at the end of the day, it's all about winning."
Thome knows with the Interleague portion of the Phillies' schedule complete, he likely won't see another similar stretch of playing time this season.
He's fine with that. He's just happy to put on the uniform every day.
"I know my time is coming to an end and that's why I think I enjoy the game more now than I ever had."
Chris Toman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.