Flacco was born on January 16, 1985, in Audubon, New Jersey. The son of Karen (née Madden) and Steve Flacco, Joe is the oldest of five boys. The Flacco family is originally from Haddon Township, New Jersey. Flacco played football, baseball, and basketball at Audubon High School and was the starting quarterback for the Green Wave. Regarded as a three-star recruit by Rivals.com, Flacco was listed as No. 39 among quarterback prospects in the class of 2003.

In 2003, as part of the Pittsburgh Panthers football team, Flacco was redshirted as a freshman for a team that went 8–5.

In 2004, Flacco was the backup quarterback behind starter Tyler Palko, who led the team to an 8–4 record. He saw action in three games against Ohio University, University of Nebraska, and the University of South Florida. Flacco threw only four passes and finished the season with one completion for 11 yards and a 25-yard punt.

Flacco transferred to the University of Delaware in 2005, but was not eligible to play that season. He saw his first full-time action during the 2006 season, throwing for 2,783 yards, 18 touchdowns and 10 interceptions. Delaware’s Fightin’ Blue Hens ultimately struggled to a 5–6 record and failed to qualify for the FCS playoffs.

In the 2007 season, Flacco led his team to an 8–3 regular season record while compiling 4,263 yards, 23 touchdowns and 5 interceptions. Arguably his best game came against Navy where he threw for 434 yards and four touchdowns. Flacco showed another solid performance in the first-ever meeting against the Delaware State Hornets in the first round of the playoffs. Behind Omar Cuff’s record-setting day, Flacco threw efficiently for 189 yards and a touchdown, leading the Blue Hens to an easy 44–7 victory. Flacco continued Delaware’s playoff run by upsetting the Northern Iowa Panthers 39–27 in the FCS quarterfinals and upsetting the Southern Illinois Salukis 20–17 the next weekend in the semifinals. Flacco threw two touchdowns to win against both the Panthers and Salukis, but went on to lose in the FCS National Championship Game to the Appalachian State Mountaineers 49–21.

At Delaware, Flacco was pessimistic about his chances to play in the NFL, and after his junior year asked his coach for permission to play baseball. The coach predicted that he would be selected in the NFL Draft, surprising Flacco. With a solid showing at the Senior Bowl and NFL Combine, Flacco solidified himself as a top five quarterback in the 2008 NFL Draft. Flacco won the long distance throw competition in ESPN’s State Farm College Football All-Star Challenge with a 74-yard throw, beating out Matt Ryan, Colt Brennan, Chad Henne, and John David Booty, later winning the Taco Bell Quarterback Scramble with a time of 15.72 seconds.

Flacco was drafted by the Ravens with the 18th overall pick in the 2008 NFL Draft after the team traded out of the eight overall selection to the 26th, then up again. The selection made Flacco the highest drafted player ever from the University of Delaware. He was also the first Division I-FCS (formerly I-AA) quarterback selected in the first round of the draft since Steve McNair went third overall to the Houston Oilers in the 1995 NFL Draft. Draft commentators initially criticized the pick as a “reach”, feeling Flacco would likely have still been available in the middle of the second round. DeCosta, however, stated waiting was too much of a gamble and that choosing Flacco in the first round was “an easy decision to make.”

On July 16, 2008, he signed a five-year contract with a maximum value of around $30 million, and $8.75 million guaranteed.

Due to a season-ending injury to incumbent starter Kyle Boller and an illness to former Heisman Trophy winner Troy Smith, Flacco became the starting quarterback in the 2008 season opener against the Cincinnati Bengals. He completed 15 of 29 passes for 129 yards, his longest pass being a 15-yard play to Derrick Mason. He threw no touchdowns and no interceptions in his debut, but he had a 38-yard rushing touchdown, which was the longest rushing touchdown by a quarterback in Ravens’ franchise history. Flacco’s touchdown put the Ravens up by 17–3; the team eventually won the game 17–10.

Flacco signed a three-year contract with Reebok as a rookie in 2008. In 2009–2010, Flacco was a spokesperson for Pizza Hut, which sold a product called “Flacco’s Favorites”. Flacco has also endorsed Nike and 1st Mariner Bank, as well as Haribo since January 2013. Also in 2013, Flacco signed an endorsement deal with McDonald’s to promote their new menu item, the Mighty Wings. Flacco has also teamed up with opendorse to promote a Zynga mobile application called “NFL Showdown: Football Manager” as well as a line of women’s apparel for Spirit Football Jersey.

In the 2009 AFC Championship Game versus the Pittsburgh Steelers, the Ravens lost 23–14, with Flacco throwing for 141 yards, being sacked a three times and throwing three picks, one being the game clincher to Troy Polamalu, who returned the pass for a touchdown.

Flacco was named the Diet Pepsi NFL Rookie of the Year in January 2009.

On December 26, 2010, with a win against the Cleveland Browns, Flacco became one of only four NFL quarterbacks to lead his team to the playoffs in all three of his first three seasons. During this game, Flacco reached 10,000 career passing yards. Ending the season, Flacco set season highs in passing yards (3,622), passing touchdowns (25), passer rating (93.6), and a season low in interceptions (10), yet set a career-high with four lost fumbles.

Flacco married his wife Dana in 2011. Together they have five children; their first child Stephen Flacco was born in June 2012, Daniel Flacco was born September 15, 2013, about an hour before the start of the Ravens’ home opener. Francis Flacco was born in January 2015 and Thomas Flacco was born in April 2018. The Flaccos’ only daughter, Evelyn Flacco, was born in September 2016.

On the NFL Network’s Top 100 Players of 2012 list, Flacco was ranked 74th by his fellow players. Despite Flacco’s success—the Ravens were in the playoffs each of his first four seasons—few saw him as among the NFL’s best quarterbacks. Before January 2015, he had never been selected for the Pro Bowl. In April 2012 Flacco was, as The New York Times later stated, “almost universally mocked” when he stated that he was the best quarterback in the NFL, superior to Brady, Peyton Manning, or Aaron Rodgers. “I don’t think I’d be very successful at my job if I didn’t feel that way”, Flacco said. In July, he turned down the Raven’s offer of a new contract, reportedly for $16 million a year, telling his agent that he believed he could improve and earn more.

On January 6, 2013, in the Wild Card Playoffs round against the Indianapolis Colts, Flacco finished the game 12 of 23 for 282 yards, two touchdowns, and a career postseason high 125.6 passer rating. The 24–9 win at home set up a Divisional round rematch with the Broncos, who entered the game as nine-point favorites after their regular season win over the Ravens, which included a 98-yard interception touchdown return by Broncos’ cornerback Chris Harris that marked the lowest point of Flacco’s season. Continuing his postseason success, Flacco played one of the best games in his career, throwing for 331 yards and 3 touchdowns. In the fourth quarter, the Ravens were down 35–28 and had one last chance to tie the game. After an incomplete pass and a 7-yard scramble, on 3rd down and 3, Flacco heaved a 70-yard touchdown pass to Jacoby Jones, sending the game into overtime. The pass has been called the “Mile High Miracle”, and “one of the greatest plays in NFL history” and drew comparisons to Roger Staubach’s “Hail Mary”, Terry Bradshaw’s “Immaculate Reception”, and Joe Montana’s “The Catch”. The Ravens would go on to win the game in double overtime 38–35 after an interception by Corey Graham from Peyton Manning and a 47-yard field goal from Justin Tucker, sending the Ravens to face the New England Patriots in the AFC Championship for a second straight year.

On January 20, 2013, Flacco and the Ravens avenged the previous year’s AFC Championship Game loss to the New England Patriots with a 28–13 win, securing their spot in the 2013 Super Bowl. Flacco threw for 240 yards and three touchdowns, notching his third straight game with a passer rating over 100. With this win, Flacco became the second NFL quarterback to defeat both Peyton Manning (with the Broncos) and Tom Brady in the same postseason since both became starting quarterbacks in 2001 (the first to do so was Mark Sanchez in 2010).

On February 3, 2013, the Ravens met the San Francisco 49ers in Super Bowl XLVII. The 49ers were a 4-point favorite to win the game, but Flacco completed 22 of 33 passes for 287 yards and three touchdowns, to lead the Ravens to a 34-31 win. Flacco’s three touchdowns in the game tied him for first place on the list of most consecutive playoff games with at least three touchdown passes (3 games). With a record fourth playoff game with a 100+ passer rating in a single postseason, Flacco was named the game’s Most Valuable Player, and was awarded a brand-new 2014 Chevrolet Corvette.

On March 4, 2013, Flacco became the highest-paid quarterback in NFL history when he signed a six-year contract worth $120.6 million. However, within months, he was surpassed by the Packers’ Aaron Rodgers and the Falcons’ Matt Ryan.

Flacco’s 2013 season was statistically the worst of his career. For the first time as an NFL quarterback, Flacco threw more interceptions than he threw touchdowns. His 22 interceptions in the season were 10 more than he had thrown in any other season. His 19 touchdowns were also his lowest since his rookie year. His passer rating was a career low 73.1, approximately 7 points lower than Flacco’s previous worst season passer rating. He was also sacked more times (48) than he was in any other season in his career. Despite all this, he also set a high in passing yards, with 3,912. The Ravens finished 8–8 for the season and Flacco missed the playoffs for the first time in his career. He was ranked 58th by his fellow players on the NFL Top 100 Players of 2014.

Flacco is the oldest of the five boys in his family. His brothers are named Mike, John, Brian, and Tom, and he also has a sister named Stephanie. His brother Mike was selected in the 31st round of the 2009 Major League Baseball draft by the Baltimore Orioles, and was the starting first baseman for the Frederick Keys, who won the 2011 Carolina League championship. He hit for a .250 average, with 5 HRs and 22 RBIs in 50 games with the Keys. The Orioles traded him to the Boston Red Sox on March 15, 2013, and rather than accepting the assignment, Flacco decided to retire from baseball. He then enrolled at the University of New Haven to play football as a tight end, and declared for the 2014 NFL Draft. Another brother, John, was a walk-on receiver with the Stanford Cardinal football team and played against the Army Black Knights football team at the 2013 Stanford vs. Army football game. Joe’s youngest brother, Tom Flacco, enrolled in 2015 at Western Michigan University, where he played quarterback for two seasons before transferring to Rutgers in 2017 and then Towson in 2018.

In their 2014 season home opener, the Ravens played against the division rival Cincinnati Bengals. The game saw the Ravens trailing practically the entire time; however, their one lead of the game came off of an 80-yard touchdown pass from Flacco to wide receiver Steve Smith Sr.. Until week nine of the 2016 season, it was the longest touchdown throw of Flacco’s career. However, the Ravens would go on to lose the game 23–16, after Bengals’ quarterback Andy Dalton connected with receiver A. J. Green for an even more stupendous 77-yard touchdown that would ultimately be the game winner. It would be the first time since 2005 that the Ravens lost their home opener. Flacco finished the game completing 35 of 62 passes and throwing for a season-high 345 passing yards along with a touchdown and an interception, with a passer rating of 71.0.

2014 saw Flacco have the best regular season of his career. He set highs in passing yards (3,986), passing touchdowns (27), times sacked (19), fumbles (5) and fumbles lost (0). He also attempted and completed the second most single-season passes in his career (344 completions out of 554 attempts) and tied his mark for second fewest interceptions thrown in a regular season (12). From Weeks 2 through 4, Flacco was not sacked at all. In 5 of Flacco’s last 6 games of the season, his passer rating was 99 or higher, with the one exception being the 25–13 loss to Houston. He led Baltimore to a 10–6 record.

On March 2, 2016, Flacco agreed to a three-year extension to remain with the team. After their Week 3 victory against the Jacksonville Jaguars, Flacco and the Ravens had accrued a 3–0 record, their best record after three games since 2009, but the Ravens would struggle down the stretch, finishing with an 8–8 record and missing the playoffs for the second straight season, the first consecutive playoff misses for the franchise since 2004–2005. Flacco posted a career-high and franchise record 4,317 passing yards, the first time he reached at least 4,000 yards, but only threw 20 touchdowns and 15 interceptions, the second highest of his career. Flacco posted a 6.4 average per attempt, tying his career worst. Flacco also completed a career-high and franchise best 436 completions, and a career-high and franchise best 672 attempts. The 672 pass attempts currently rank fifth most in a single season in league history. The longest TD pass of his career came in week nine of the 2016 season against the Pittsburgh Steelers. In the first quarter of the game, Flacco’s short pass to Mike Wallace went for a 95-yard touchdown, and is the longest pass play in Ravens history.

On July 26, 2017, it was revealed that Flacco was diagnosed with a back injury, ruling him out for 3–6 weeks of training camp. During Thursday Night Football against the Miami Dolphins in Week 8, Flacco suffered a concussion after a hit from Kiko Alonso. Flacco left the game, and was relieved by Ryan Mallett for the rest. The Ravens nevertheless won 40–0. The Ravens finished the season with a 9–7 record and just missed the playoffs after losing 31–27 to the Cincinnati Bengals in Week 17, allowing the Buffalo Bills to qualify for the postseason. Flacco finished the season starting all 16 games, throwing for 3,141 yards and 18 touchdowns with 13 interceptions.

In a survey by Fanatics in January 2017, Flacco was elected as the most attractive player in the NFL.

On September 9, 2018, Flacco threw for three touchdowns and had a passer rating of 121.7 in the season-opening blowout against the Buffalo Bills 47–3. The next week, on Thursday Night Football, he recorded 376 yards and two touchdowns along with two interceptions against the Cincinnati Bengals, losing his seventh game in Cincinnati. He fumbled the ball in the fourth quarter, ending the chance for a comeback.

On February 13, 2019, the Ravens agreed to trade Flacco to the Denver Broncos in exchange for their fourth-round pick in the 2019 NFL Draft. The deal became official on March 13, 2019.

On March 19, 2020, the Broncos released Flacco with a failed physical designation. He underwent neck surgery in April 2020.

On May 27, 2020, Joe Flacco signed a one-year, $1.5 million deal with the New York Jets as the backup quarterback to 2018 first-round pick Sam Darnold. He was placed on the active/physically unable to perform list at the start of training camp on July 30, 2020. He was activated from the list at the start of the regular season on September 5, 2020. Flacco made his Jets debut when he briefly entered the Thursday Night Football Week 4 matchup on October 2 after starter Sam Darnold injured his shoulder late in the first quarter. He played only four snaps, going 2 for 2 for 16 yards before Darnold returned in the second quarter in the 28-37 loss to the Denver Broncos. On October 7, 2020, Flacco was named the starter against the Arizona Cardinals due to Darnold’s shoulder injury. Making his first start with the Jets in Week against the Arizona Cardinals, Flacco finished with 195 passing yards and a touchdown as the Jets lost 10–30. In the next game against the Miami Dolphins, Flacco finished with 186 passing yards, an interception, and a fumble that he recovered. The Jets again lost 0-24, the first time Flacco had ever been shut out in his NFL career. In Week 9 against the New England Patriots on Monday Night Football, Flacco was again thrust into the starting role after Darnold re-injured his shoulder the previous week. He had his best game of the season, going 18 for 25 for 262 yards and three touchdowns while also passing Joe Montana for 20th on the all-time passing yards list. Unfortunately, he also threw a costly fourth quarter interception that allowed the Patriots to rally and eventually beat the Jets by a score of 30-27.