He had a modest minor league career reaching AAA by 1992 after hitting 17 HRs the previous year. In 1992 he was sent along with Rickey Bones to the Milwaukee Brewers for Gary Sheffield.
Valentin made his debut with the Brewers as a September call up late in the year. After two seasons up & down to the big leagues he became the Brewers regular short stop by 1995. It took time to get adjusted, as he wasn’t the slickest fielder around, leading the league in errors in 1994,1996 & later again in 2000.
His best season in Milwaukee was 1996 hitting 24 HRs with 33 doubles &95 RBIs batting .259. Valentin was a free swinger which led to a lot of strike outs. He struck out 145 times that seasonand would K over 100 times seven times in his career.
He hit 16 or more HRs over the next two seasons but never hit above .230. After eight seasons in Milwaukee he was sent about a two hours south to the city of Chicago, to the White Sox in exchange for Jaime Navarro and John Snyder.
In Chicago Valentin’s power numbers increased as many players did during the so called "steroid or juiced ball era" of baseball history. Valentin hit 25 or more HRs in each of his five seasons there, with four seasons of 70 plus RBIs. In 2000 he had career highs in hits (155) RBIs (92) runs scored (107) doubles (37) & stolen bases (19) playing for Jerry Manuel & the first place White Sox. In his first post season he batted .300 in the ALDS loss to the Seattle Mariners.
In 2001 he hit 28 HRs matching that mark again in 2003. In 2004 he hit his career high 30 HRs driving in 70 runs but batted a weak .216 with 139 strikeouts (5th most in the league). As he got older, he had less range at short and moved around the diamond playing some outfield & third base.
In 2005 he signed as a free agent with the Los Angeles Dodgers, but a nagging knee injury limited him to only 56 games and a .170 batting average. His contract wasn’t renewed and the New York Mets gave him a chance.
He surprised everyone in Spring Training 2006 and made the Mets squad going north. He became the Mets steady everyday second baseman taking the job from Kazo Matsui in mid May & was soon traded away. Valentin was solid only making six errors all season at second, posting a .978 fielding percentage, the best of his career.
He had a strong May starting with a four hit game against his old Milwaukee Brewers team mates on May 14th. In that game he hit a two run HR off Chris Demaria & had a four RBI day. He drove in runs in three straight games in that series & then hit a HR on the next trip to St. Louis.
Later that month he drove in two runs in a game in Florida & then also had a big series against the Arizona Diamondbacks at Shea Stadium. In the Mets two out of three series win, Valentin had six hits, with a pair of HRs, a double & four RBIs.
In the last week of June he had seven hits during inter league play, hitting HRs in Toronto & Boston. That week he also had five RBIs. In the second game of a July 8th double header, Valentin had a huge day in the Mets 17-3 win at Shea Stadium. He hit a grand slam HR off Ricky Nolasco in the 1st inning then added a bases loaded, base clearing triple in the second.
On July 21st Valentin hit another grand slam, this time in a home game against the Houston Astros in a 7-0 Mets win. On July 26th he drove in the only run of the game with a walk off game winning single off former Met Glendon Rusch.
In the month Valentin drove in 17 runs & raised his average to .290. On August 12th he singled in the top of the 7th inning, breaking a 4-4 tie in Washington. The Mets went on to win the game 6-4. The next day he tied up the game with when he reached on an error scoring David Wright.
On August 30th he hit a pair of HRs in Colorado helping the Mets to an 11-3 win. On September 1st he broke up a 6-6 tie in Houston with a double scoring David Wright. The Mets went on to an 8-6 loss. On September 7th he hit another HR driving in two runs in the Mets 7-0 win over the Dodgers at Shea.
On September 18th, 2006 the Mets clinched the NL Eastern Division at Shea Stadium. That night it was Valentin who had a big night, hitting two HRs driving in three runs helping the Mets beat the Marlins 4-0.
In the 2006 Mets NL Eastern Division Champion year, Valentin batted .271, with 18 HRs, 24 doubles, 62 RBIs 56 runs scored a.330 on base % and three triples. His veteran leadership added to the club house and his moustache made for one of the best in Mets history.
Post Season: Valentin played every game in the ’06 post season, starting out by going hitless against the Los Angeles Dodgers in the NLDS.
In Game #4 of the NLCS against the St. Louis Cardinals Valentin had two hits, with walk, & three RBIs.
In the top of the 6th he doubled with the bases loaded putting the game away with a 9-3 Mets lead, they would go on to win 12-5 evening the Series at two games each.
In Game #4 he drove in the first two runs of the game with a double off Jeff Weaver. They were the only runs Weaver allowed as the Cards rallied to win the game 4-2 at Busch Stadium. Overall he went 6-24 in the NLCS (.250) with two doubles & Five RBIs.
He returned in 2007 and gave up his uniform number 18 for Moises Alou, Valentin took #22. Age began to catch up with him; on July 20th he fouled a ball off his right leg breaking his fibula & ending his season.
In 51 games in 2007 he only hit .241 with 3 HRs 11 doubles a .302 on base % & 18 RBIs. The next season the Mets gave him a minor league contact, but at age 38 he couldn’t come back from the injuries. He did not make the ball club & retired.
His Mets numbers in two seasons are 21 HRs 80 RBIs 35 doubles a .322 on base % & a .262 average in 188 games played. Overall In his 16 year career he hit 249 HRs ranking him in the top 200 All Time. He hit .243 1348 hits, with 302 doubles, 41 triples, 872 runs scored a .321 on base % & 816 RBIs in 1678 games played.
On the field he played 181 games at third making 31 errors (.936 %) 158 games at second with 12 errors (.985%) & 54 games in the outfield (.990%).
Retirement: In 2004 Valentin purchased one of the most historic baseball franchises in Puerto Rico, relocating the Santurce Crabbers team to his native town. After attendance fell off he brought the team back to their original home.
He also has played with the team at second base in his forties. In 2012 he was named manager of the A ball Fort Wayne Caps, an affiliate of the San Diego Padres.