His mother hoped to direct Pagán's high level of energy into a constructive outlet, and encouraged the child to play the sport.

"[1] Pagán was given the nickname "El Caballo Loco" (Spanish for "Crazy Horse") by his teammates during his early years as player for Las Lomas Potros in Rio Piedras, Puerto Rico manage by Willie Ronda.

Pagán made his major league debut with the Cubs on April 3, 2006, but was injured early in the season and missed two and a half months.

On July 2, 2006, his 25th birthday, Pagán hit his first and second career home runs in a game against the crosstown rival Chicago White Sox.

Pagán then became a regular outfielder and enjoyed modest success until his season was cut short due to a bout with colitis.

On June 2, 2007, Pagán was called out at third base trying to move up after a wild pitch, which led to Lou Piniella's most infamous meltdown.

Chicago general manager Jim Hendry was afraid Pagán would not be ready to start the 2008 season on time, thus prompting the trade to New York.

On May 7, 2008, Pagán made a spectacular catch in foul territory, falling into the stands, in Los Angeles against the Dodgers while playing left field.

On August 1, 2009, Pagán hit a grand slam for his first home run since mid-2007 and first for the New York Mets, leading to the win over the Arizona Diamondbacks.

Pagán was not the 2010 Opening Day center fielder (the nod went to newly acquired Gary Matthews, Jr.), despite making a strong case in 2009.

Due to Matthew's poor performance, Pagán shortly found himself back in the starting lineup and quickly established himself as a quality everyday player.

On May 19, 2010, Pagán started a triple play from center field in the 5th inning against the Washington Nationals on a ball hit by Cristian Guzmán.

With the uncertainty of Beltran's health as well as his range, the speedy Pagán was inserted into the starting center fielder position for the 2011 season.

On July 20, Pagán hit a 10th-inning walk-off home run off the railing of the second deck (named the Pepsi Porch in Citi Field) against closer Fernando Salas to help the Mets win 6-5 over the St. Louis Cardinals.

[6] On January 16, 2012, Pagán signed a one-year deal with the San Francisco Giants worth $4.85 million to avoid his final year of arbitration.

[10] On May 25, with the Giants trailing 5-4 in the bottom of the tenth inning versus the Colorado Rockies, Pagán hit a walk-off inside-the-park home run off of reliever Rafael Betancourt.

[12] On June 20, Pagán re-injured his left hamstring while on rehab assignment with the Class-A Advanced San Jose Giants against the Stockton Ports.

[13] On June 25, he underwent surgery to repair a torn tendon in his left hamstring, and he did not join the Giants again until August 30, missing 82 games.

[19] After three injury-riddled seasons and advanced metrics that suggested worsening defense, the Giants moved Pagán to left field to make way for Denard Span.

Pagán finished the season with a .277 batting average and a career-high 12 home runs in 495 at-bats, while seeing the highest percentage of fastballs of all MLB hitters (66.7%).

[21] On April 19, 2017, while a free agent, Pagan announced that he would not be playing for the 2017 season, stating that he had dissatisfaction with contract offers and wanted to spend some time with his family.

[28] In May 2019, Pagán was rescued out at sea near Cibuco Beach in Vega Alta, Puerto Rico, when the boat he and fellow professional baseball player Orlando Román were in capsized.