He played college football for the Pittsburgh Panthers and the Delaware Fightin' Blue Hens and was selected by the Baltimore Ravens in the first round of the 2008 NFL draft.

[2] A decline in performance and a hip injury resulted in Flacco losing his starting position to Lamar Jackson; he was subsequently traded to the Denver Broncos in 2019.

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.

[23] Members of the Baltimore Ravens' front office, having attended Flacco's workout at the Senior Bowl, were impressed by his ability to throw in rainy and windy conditions.

Ravens assistant general manager Eric DeCosta later stated the workouts left him confident Flacco could succeed in the late-season conditions in Pittsburgh, Cleveland, and Cincinnati.

[26] 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.

[31] Shaun King, Ben Roethlisberger, Russell Wilson, Mark Sanchez, Brock Purdy, C. J. Stroud, and most recently Jayden Daniels are the only other rookie quarterbacks to ever win their debut playoff game.

Flacco made tight-window throws to Todd Heap and Mark Clayton on the go ahead scoring drive in the fourth quarter to set up a game-winning field goal from Matt Stover.

He then connected with prized offseason acquisition wide receiver Anquan Boldin for three touchdowns while throwing no interceptions and accumulating a passer rating of 128.7.

[42] In defeating the Chiefs (30–7) in the Wild Card Round of the 2010–11 NFL playoffs, Flacco completed 25 of 34 passes for 265 yards, two touchdowns, and no interceptions, for a post-season-high passer rating of 115.4.

He went 27-of-48 with a season-high 389 yards through the air and three touchdowns, all in the first quarter and to rookie wideout Torrey Smith (who actually finished the game with a five catch, 152-yard breakout performance).

Despite Flacco's performance, the Patriots advanced to the Super Bowl when a potential game-tying field goal attempt by kicker Billy Cundiff was shanked wide left in the final minute of the game.

"[63] On January 6, 2013, in the Wild Card 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 pass has been called the "Mile High Miracle", and "one of the greatest plays in NFL history"[66] and drew comparisons to the "Hail Mary", "Immaculate Reception", and "The Catch".

[67] On January 20, 2013, Flacco and the Ravens avenged the previous year's AFC Championship Game loss to the Patriots with a 28–13 win, securing their spot in the 2013 Super Bowl.

[72] These accomplishments gave Flacco what The New York Times stated, "might be the best start to a player's free agency in the history of professional sports", as his contract expired after the season.

[85] In Week 17, the Ravens took on the Browns in Baltimore with a chance to clinch the final playoff seed, but they needed a win and for the Kansas City Chiefs to beat or tie the San Diego Chargers.

Trailing 6–10 in the fourth quarter, Flacco stepped up big time, heaving a deep pass to Torrey Smith, who caught the ball at the Browns' 16-yard line.

[88] In the Divisional round of the playoffs, the Ravens traveled to Foxborough to take on the number-one seeded Patriots, where they lost to the eventual Super Bowl champions in a 31–35 shootout, despite two back-to-back 14-point leads.

[89] For his efforts during the 2014 season, Flacco was invited to the 2015 Pro Bowl as an alternate but turned down the opportunity to play because his wife, Dana, was due to give birth to the couple's third child the same month.

However, while trying to rally the Ravens back at the end of the game, he was picked off by cornerback Neiko Thorpe with less than 30 seconds left, giving the Raiders a 37–33 upset win.

On the final drive of the Ravens' Week 11 game versus the Rams, Flacco suffered a torn ACL and MCL, ending his 2015 season.

[94] He stayed in the game to complete the drive and put Baltimore into a position to kick the game-winning field goal, giving the Ravens a 16–13 win at home.

[101] The Ravens finished the season with a 9–7 record and just missed the playoffs after losing 31–27 to the Bengals in Week 17, allowing the Buffalo Bills to qualify for the postseason.

[104] After suffering a hip injury during a Week 9 loss to the Steelers,[105] Flacco was replaced by rookie Lamar Jackson as the Ravens' starting quarterback.

With 31 seconds left in the game, Flacco threw a seven-yard touchdown pass and a two-point conversion, both to wide receiver Emmanuel Sanders to take a 14–13 lead.

[124] Flacco made his Jets debut when he briefly entered the Week 4 matchup on October 2 after starter Sam Darnold injured his shoulder late in the first quarter.

[165] With Richardson struggling, the Colts announced on October 30 that Flacco would be the starter moving forward, beginning in their Week 9 matchup against the Minnesota Vikings.

[193] While playing for the Ravens, Flacco lived in the Baltimore suburb of Reisterstown, Maryland, until selling his home for $1.6 million in 2019 after being traded to the Denver Broncos.

[197][198] 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.