Tejada spent his first six seasons in MLB with the Athletics, where he began a streak of 1,152 consecutive games that ended with the Orioles on June 22, 2007.

[4][5] Tejada grew up in extreme poverty in Baní, a city about 40 miles (65 km) southwest of Santo Domingo, capital of the Dominican Republic.

He grew up idolizing the Baltimore Orioles shortstop Cal Ripken Jr.[6] Tejada developed quickly into a top-notch prospect, showing early signs of power.

After a solid 87-win campaign in 1999, Tejada and a core of young players led their A's to their first American League Western Division title in eight years in 2000.

Bolstered by an American League MVP-winning performance by first baseman Jason Giambi, and aided by Tejada's .275 average and 30 home runs, the A's won 91 games.

With the departure of Jason Giambi to the New York Yankees during the offseason, and a leg injury to slugger Jermaine Dye, the A's lost two of their key offensive players.

Tejada contributed one-out, game-winning hits in the 18th and 19th games of that run: a three-run homer off Minnesota Twins closer Eddie Guardado for a 7–5 victory and a bases-loaded single against Kansas City Royals reliever Jason Grimsley to break a 6–6 tie.

Tejada was known for his public display of anger toward Boston starting pitcher Derek Lowe at the series' conclusion for what he perceived as obscene gestures.

The A's elected not to re-sign the free agent, citing budget concerns and a young Bobby Crosby coming through the system, so Tejada signed a six-year, $72 million deal with the Baltimore Orioles during the offseason.

Tejada finished the 2004 season with 34 home runs and an MLB-leading 150 RBIs, and won his first career Silver Slugger Award.

In his first All-Star start, Tejada hit a solo home run against John Smoltz of the Atlanta Braves, had a sacrifice RBI and was part of an all-Oriole double play with teammate Brian Roberts.

In the winter of 2005, Tejada asked the Orioles for a trade, citing unhappiness with the team's direction, while commenting briefly on his alleged non-involvement in Palmeiro's steroid scandal.

On June 22 he was placed on the disabled list, ending his streak at 1,152 consecutive games, the fifth longest in Major League history, behind Cal Ripken Jr. (2,632), Lou Gehrig (2,130), Everett Scott (1,307), and Steve Garvey (1,217).

[12] On December 12, 2007, Tejada was dealt to the Houston Astros for five players, including SP Troy Patton, OF Luke Scott, RP Dennis Sarfate and RP/SP Matt Albers.

In the 2008 All-Star Game Tejada singled leading off the top of the eighth stole second with one out and advanced to third on a throwing error and scored on Padres' first baseman Adrián González's sacrifice fly.

[17] On July 29, the Orioles traded Tejada to the San Diego Padres for minor league pitcher Wynn Pelzer.

[23][24] He failed to make the Major League club's roster however, and played for their Triple-A affiliate Norfolk team until requesting his outright release, which was granted on June 25, 2012.

According to the Associated Press, he was to earn $1.1 million plus performance incentives worth an additional $400,000 if he made the Royals Major League 40-man roster for 2013.

[25] On August 17, 2013, Tejada received a 105-game suspension from Major League Baseball following two positive drug tests for amphetamines.

Tejada claimed to have been in the process of re-applying for a therapeutic use exemption as he used the drug to treat a medical condition, but he chose not to appeal the decision.

Due to a calf injury suffered prior to the ban, Tejada was not likely to play in the remaining 41 games of the 2013 season anyway, though they counted toward his suspension.

"[32] In José Canseco's 2005 book, Juiced: Wild Times, Rampant 'Roids, Smash Hits & How Baseball Got Big, he mentioned that he believed Tejada might have taken steroids.

[1] A report surfaced on January 15, 2008, stating that Rep. Henry Waxman had asked the Justice Department to investigate whether Tejada was truthful when speaking to the House committee when being interviewed in 2005 regarding possible connections to Rafael Palmeiro.