Jennings was born in Compton, California to Alice Knox. He has a half brother named Terrence Phillips. His father died when he was young.
Jennings attended Dominguez High School in Compton, California for his freshman and sophomore year. Before his junior year, Jennings transferred to powerhouse Oak Hill Academy in Mouth of Wilson, Virginia. In his last year of high school, Jennings averaged 35.5 points per game and set the school record for points in a season (1,312). This performance earned him some of high school basketball's most prestigious awards: the 2008 Naismith Prep Player of the Year Award, 2007–08 Gatorade Player of the Year (Virginia), 2008 Parade Magazine Player of the Year and 2008 EA Sports Player of the Year. He led his 2006–2007 team to a 41–1 record and the top ranking in the USA Today Super 25 list of high school teams. He was rated as the nation's #1 high school basketball prospect in the class of 2008 by Scout.com, the #1 prospect in the ESPNU 150, and the #4 prospect by Rivals.com.
In August 2006, he chose to attend USC. On April 24, 2007, he decided instead to join the Arizona Wildcats, citing Arizona's quality academic faculty and his desire to play with Jerryd Bayless. (Bayless left after one season to enter the 2008 NBA draft.) In November 2007, SLAM Magazine's third edition of PUNKS featured Jennings on the cover along with three other top-rated high school guards (Jrue Holiday, Tyreke Evans and Lance Stephenson).
In June 2008, Jennings attended the premiere of Beastie Boys' Adam Yauch's basketball movie Gunnin' For That #1 Spot at the Magic Johnson Theater in Harlem. Also attending were Kevin Love, Stephenson, Brook Lopez, Robin Lopez, DJ Augustin, and Yauch. The film follows eight top high school players—including Jennings—from their hometowns to New York City, for the 2006 Elite 24 at Rucker Park.
In June 2008, Jennings announced that he was considering becoming the first American to skip college to play professionally in the Euroleague. The National Basketball Association requires players to be at least 19 years old and one year removed from high school before entering the league, meaning that Jennings could not enter the 2008 NBA draft. Jennings declared that his goal was to play in the NBA and that playing overseas instead of at an American college could be his best route to gain experience and make money until he was eligible to join the NBA.
On July 16, 2008, Jennings signed with Lottomatica Roma of the Italian Serie A. The contract he signed with Roma was for $1.65 million net income guaranteed. After earning the contract with Lottomatica, Under Armour gave Jennings a $2 million contract to showcase their products in the Euroleague. Jennings was the first player to play for a European team rather than play for a college basketball team since the NBA's age restriction rule was implemented.
In the Italian Serie A 2008–09 season, Jennings averaged in 27 games, 5.5 points, 1.6 rebounds, 2.2 assists, and 1.5 steals in 17.0 minutes per game. He shot 35.1 percent from the field and 20.7 percent from 3 point range in Serie A play. In 16 Euroleague games, Jennings averaged 7.6 points, 1.6 rebounds, 1.6 assists, and 1.2 steals in 19.6 minutes per game. In the Euroleague he shot 38.7 percent from the field and 26.8 percent from 3 point range.
Jennings was selected tenth overall by the Milwaukee Bucks in the 2009 NBA draft. He became the first player who skipped college to play professional basketball in Europe to be drafted by an NBA team. Jennings also made a notable appearance at the draft. He had initially decided not to attend the draft and preferred to be at a family function during the draft. After he was drafted by the Bucks, he left the family function and headed to Madison Square Garden. He later came out on stage after the 14th pick was announced to have his picture taken with NBA commissioner David Stern, just like all drafted players who attend the draft.
During Jennings's NBA regular season debut on October 30, 2009, against the Philadelphia 76ers, he recorded 17 points, 9 rebounds, 9 assists, and hit 2 three-point shots, and played 34 minutes. In his second game, on October 31, 2009, against the Detroit Pistons, also his debut in Bradley Center, Jennings scored 16 points during the third quarter and a team-high 24 points for the game to lead the Bucks to a victory.
On November 14, 2009, in just his seventh game in the NBA, Jennings scored 55 points in a win over the Golden State Warriors. After going scoreless in the first quarter, Jennings erupted for 29 points in the 3rd quarter. He broke the team record for most points by a rookie previously set by Kareem Abdul-Jabbar in 1970, and was the most points scored by a rookie since Earl "The Pearl" Monroe scored 56 in 1968. He became the youngest player to ever score 55, collecting the second-highest total for a player under 21, behind only LeBron James's 56 points in March 2005, and the second-most points scored by a Milwaukee Buck (behind Michael Redd's 57 in 2006).
During All-Star Weekend, he competed in the Skills Challenge. Jennings started all 82 games as a rookie, and led the Bucks to the playoffs for the first time in four seasons, where they lost to the Atlanta Hawks in seven games.
On October 30, 2010, Jennings recorded his first ever triple double, with 20 points, 10 rebounds, and 10 assists in a win against the Charlotte Bobcats. On December 15, 2010, Jennings broke his left foot against the San Antonio Spurs when he came down awkwardly on his ankle. Despite the injury, he finished the game and played 30 minutes the following game against the Jazz. The team later confirmed that Jennings would miss the next four to six weeks as he recovered from surgery to repair a fracture in his left foot. He had been averaging 18.7 points per game before the injury.
Jennings missed a total of 19 games with the injury before returning on January 29, 2011 in a 91–81 home victory over the New Jersey Nets. He scored a season-high 37 points against the New York Knicks at Madison Square Garden on March 25, 2011. He considered the game his revenge of sorts, as he wanted to play in New York but the Knicks passed over him in the draft and selected Jordan Hill.
With Milwaukee struggling in Jennings absence and his slow return to form after the injury, the Bucks missed the playoffs.
With the NBA beginning the 2011–12 season in a lockout, Jennings played in the Drew League to stay in shape and prepare for the season. With the schedule shortened after the lockout ended, Jennings played and started in all of the 66 games. He recorded career highs with 19.1 points and 1.6 steals in 35.3 minutes per game.
Similar to the previous season, his season–high in points came in Madison Square Garden against the Knicks; this time, Jennings scored 36 points in a 100–86 win.
The Bucks again missed the NBA playoffs, finishing in ninth place in the East.
With Jennings set to hit restricted free agency at the end of the 2012–13 season, he sought a long–term contract, saying he would not return to Milwaukee if they did not work out an agreement. Despite this, he reportedly turned down a four-year contract extension (worth $40 million), despite multiple players from his draft class receiving contract extensions, including Blake Griffin and James Harden.
During the Bucks home opener against the Cleveland Cavaliers, Jennings made a game-winning three at the buzzer to win the game for the Bucks. Jennings teamed up with Monta Ellis, who was acquired at the 2012 trade deadline, to form one of the NBA's most potent backcourts that year.
With improved play under new coach Jim Boylan, Milwaukee returned to the playoffs for the first time since Jennings was a rookie. However they lost to the defending-champion Miami Heat in the opening round, who went on to repeat as champions.
On July 31, 2013, Jennings was signed-and-traded to the Detroit Pistons in exchange for Brandon Knight, Khris Middleton and Viacheslav Kravtsov. He agreed to a three year contract, reportedly worth $25 million.
On December 5, 2013, Jennings returned to Milwaukee in a 105–98 win over the Bucks, finishing with 17 points and 11 assists. On January 11, 2014, in a game against the Phoenix Suns, Jennings recorded 16 assists in the first half, tying Isiah Thomas' franchise record for the most assists in a half.
On January 16, 2015, Jennings scored a season-high 37 points on 10-of-23 shooting in the 98-96 win over the Indiana Pacers. Five days later, he recorded 24 points and a career-high 21 assists in the 128–118 win over the Orlando Magic, becoming the first player to record a 20-point, 20-assist game in the NBA since Steve Nash in 2009.
On January 25, 2015, Jennings was ruled out for the rest of the season due to a ruptured left Achilles tendon that required surgery. On the season, he averaged 15.4 points, 6.6 assists and 1.1 steals in 41 games.
On December 17, 2015, Jennings voluntarily reported to the Grand Rapids Drive, the Pistons' D-League affiliate, on a rehab assignment. In his debut with the Drive on December 19, Jennings recorded a double-double with 11 points and 12 assists in 27 minutes against the Iowa Energy. He was recalled by the Pistons the next day. On December 29, Jennings made his first appearance of the season for the Pistons, scoring seven points and adding four assists in 16 minutes off the bench against the New York Knicks. He had a season-best game in his fourth game back from injury, recording 17 points and 6 assists off the bench on January 4 in a 115–89 win over the Orlando Magic.
On February 16, 2016, Jennings was traded to the Orlando Magic, along with Ersan İlyasova, in exchange for Tobias Harris. The next day, he made his debut with the Magic in a 110–104 win over the Dallas Mavericks, recording 18 points, three rebounds and four assists in 25 minutes.
On July 8, 2016, Jennings signed with the New York Knicks. He made his debut for the Knicks in the team's season opener on October 25, 2016 against the Cleveland Cavaliers. In 21 minutes off the bench, he recorded seven points, three rebounds and five assists in a 117–88 loss. On December 11, he scored 15 of his season high-tying 19 points in the fourth quarter of the Knicks' 118–112 win over the Los Angeles Lakers. On December 31, he set a new season high with 32 points in a 129–122 loss to the Houston Rockets. On February 27, 2017, he was waived by the Knicks.
On March 1, 2017, Jennings signed with the Washington Wizards.
On July 28, 2017, Jennings announced that he had accepted a one-year, $1.5 million deal to play for the Shanxi Brave Dragons of the Chinese Basketball Association.
Note: The EuroLeague is not the only competition in which the player participated for the team during the season. He also played in domestic competition, and regional competition if applicable.2005 Press Telegram Freshmen Player of the Year
Named the Most Valuable Player of the 2006 Les Schwab Invitational Tournament
Co-MVP of the 2007 Elite 24 Hoops Classic (with Tyreke Evans)
2007 Third-team Parade All-American
2007 Las Vegas Easter Classic Most Valuable Player
2007 NBAPS Top 100 High School Camp Best Playmaker
2007 The Goazcats.com Showdown Most Valuable Player
2008 Naismith Male Player of the year
2007–08 Gatorade Player of the year Virginia
2008 Parade Magazine Player of the Year
2008 EA SPORTS National Player of the Year
2008 McDonald's All-American
2008 First-team Parade All-American
2007–08 MaxPreps National Player of the Year
2008 Jordan Brand Classic Most Valuable Player for the East
No. 1 rated senior by ESPN (2008)
No. 1 rated senior by Van Coleman Hoopmaster (2008)
No. 1 rated senior by Clark Franics Hoopscoop (2007 and 2008)
No. 1 rated senior by Dave Telep Scout.com 2008
Eastern Conference NBA Rookie of the Month (Oct./Nov. 2009)
Eastern Conference NBA Rookie of the Month (Dec. 2009)
Eastern Conference NBA Rookie of the Month (Jan. 2010)
2009–10 NBA 1st Team All-Rookie (unanimous selection)
NBA recordsYoungest player in NBA history to score 50 or more points in a game
20 years, 52 days
55, Milwaukee Bucks vs. Golden State Warriors, November 14, 2009
Ranks 2nd in NBA history in:Field goals made, quarter
12, third quarter, Milwaukee Bucks vs. Golden State Warriors, November 14, 2009
Tied with six other players who have made 12 field goals in a quarter
NBA record is held by David Thompson and Klay Thompson, tied with 13
Milwaukee Bucks franchise recordsPoints, game, rookie: 55, vs. Golden State Warriors, November 14, 2009
Tied with multiple players for 4th highest tally in NBA history by a rookie
Field goals made, game, rookie: 21, vs. Golden State Warriors, November 14, 2009
FranchisePoints, half: 45, second half, vs. Golden State Warriors, November 14, 2009
Points, quarter: 29, third quarter, vs. Golden State Warriors, November 14, 2009
Field goals made, half: 17, second half, vs. Golden State Warriors, November 14, 2009
Field goals made, quarter: 12, third quarter, vs. Golden State Warriors, November 14, 2009
After the 2009 L'Aquila earthquake, while he was playing in Italy in April 2009, Jennings donated $50,000 to support victims.
Jennings participated in Steve Nash's Showdown in Chinatown charity soccer match in New York City on June 24, 2010.
Jennings, specifically his tense relationship with Milwaukee coach Scott Skiles, was the subject of the song "Chartjunk" by indie rock band Stephen Malkmus and the Jicks in 2014.