Adu was born and raised in the port city of Tema, Ghana, where he played soccer against men three times his age. In 1997, when he was eight, his mother, Emelia Adu, won the Green Card Lottery, and his family moved to Rockville, Maryland, in the United States, where he attended Sequoyah Elementary School. In February 2003, he became a U.S. citizen. Soon after arriving in the United States, he was discovered by a local soccer coach and began playing with boys several years older. Adu attended The Heights School, a private school in Potomac, Maryland, for several years.

At the age of 14, Adu became the youngest American ever to sign a major league professional contract in any team sport when he was chosen by D.C. United as the number one overall pick in the 2004 MLS SuperDraft. In order to allow Adu to play close to home, MLS assigned him to D.C. United on November 18, 2003, working a deal with the Dallas Burn, who owned the top pick in the 2004 MLS SuperDraft. Dallas was compensated with a player allocation. Having already signed with D.C. United, Adu effectively became the first player selected in that draft, two months before it officially took place. D.C. United had previously signed American youngsters Bobby Convey in 2000, and Santino Quaranta in 2001—both aged 16 and the youngest player in MLS at the times of being drafted.

Adu was criticized from a number of different angles in his first season as a professional. Some commentators have suggested that Adu was too young to be playing professionally and that he needed more time to develop mentally and physically amongst players his own age. In his second season, he was suspended for one game after he complained about his playing time in the media. During November 2006, Adu had a trial with English club Manchester United for a fortnight, but he was not able to gain a work permit and so could not play in any competitive games, and only trained with several players from the Manchester United academy during his two weeks. Continued development, especially of his defensive skills, helped Adu become a starting midfielder during the 2006 season. In addition, Adu had been chosen to take spot kicks during D.C. United’s two penalty shootouts—scoring on both attempts. He was selected to the MLS All-Star team twice, once as a commissioner’s choice and once as the coach’s. He was selected to the MLS 2006 semifinals Best XI by Soccer America magazine. In 2005, he was nominated for FIFPRO Young player of the year.

On December 11, 2006, D.C. United traded Adu and goalkeeper Nick Rimando to Real Salt Lake in exchange for a major allocation, goalkeeper Jay Nolly, and future considerations. Adu made his debut for Real Salt Lake on April 7, 2007, playing the full 90 minutes in a 2–2 draw with FC Dallas. He scored his first goal for the club on May 20, 2007, converting a penalty kick in the 68th minute of a 2–1 loss to FC Dallas. Adu went on to score his second goal with Salt Lake from another penalty in a 1–1 draw in an exhibition against Boca Juniors. Adu was also captain of the U-20 United States men’s national team in the 2007 FIFA U-20 World Cup, where he made an impressive showing. After the conclusion of that tournament, Benfica of the Portuguese Liga secured Adu’s rights from MLS for a transfer fee of $2 million.

Adu was called into the national team’s training camp by coach Bruce Arena in January 2006 for a friendly match against Canada. Adu was capped in that game on January 22 (at 16 years and 234 days), becoming the youngest player to debut with the U.S. national team when he replaced an injured Eddie Johnson in the 81st minute at Torero Stadium in San Diego, California.

On July 28, 2007, Adu opted out of playing for Real in their regular-season match, and later that day, boarded a plane to Portugal to negotiate with Benfica. On July 30, Benfica issued an official statement announcing that Adu had been transferred to their club. The following day, the signing was completed and he trained with the team in Lisbon. Adu cost Benfica US$2 million. On August 14, 2007, Adu made his debut with Benfica against Copenhagen in a UEFA Champions League qualifying match, coming into the game in the 37th minute as a substitute.

Adu captained the United States U-20 men’s national team in January 2007 as it qualified for the 2007 FIFA U-20 World Cup in Canada. By playing in the 2007 tournament finals, held from June to July, Adu became only the second player in the world to play in three FIFA U-20 World Cups. On July 3, 2007, Adu scored a hat-trick in U.S.’s 6–1 victory over Poland in the group stage of the tournament. This accomplishment made him the first player to ever score a hat trick in both the U-20 and U-17 World Cups. In the following 2–1 victory over Brazil Adu assisted on both USA goals by Jozy Altidore.

Adu made his first start for the senior international team in a friendly against South Africa on November 17, 2007.

In July 2008, Adu joined French club AS Monaco on a season-long loan, with an option to join the Ligue 1 club permanently at the end of the deal, an option that was eventually declined by Monaco. In July 2009, after participating in the Gold Cup game against Honduras, he returned to Benfica for training.

Adu was named to the 18-man squad that represented the United States in the 2008 Summer Olympics in Beijing. Adu played in the first two games of group play against Japan and Netherlands. He assisted on a Sacha Kljestan goal in the Netherlands match, but he, as well as teammate Michael Bradley, was then suspended for the final game of group play against Nigeria after each player earned his second yellow card of group play late in the Netherlands match. The US team was eliminated from the Olympics after falling to Nigeria.

Adu was loaned to Portuguese club Belenenses for the 2009–10 season. On October 13, Adu made his first league start in Portugal, a 1–0 loss to Nacional. Adu was injured just before half time, and was substituted. In December 2009 Adu’s season long loan was cut short.

Adu signed a 12-month loan deal with Greek side Aris in January 2010, joining fellow USA international Eddie Johnson. Assigned squad number 11, Adu made his debut on January 31 and assisted a 90th-minute goal by Johnson. He scored his first goal on February 14 against Ergotelis.

On February 1, 2011, Adu moved to Turkish Second Division side Çaykur Rizespor on what was his fourth loan deal away from Benfica. Adu made his debut for his new club on February 20. He set up teammates Mithat Yaşar and Gökhan Kaba with goals in a 3–0 win over Orduspor and was named Man of the Match. Two weeks later, on March 6, Adu scored his first goal for his new club in a 1–0 win over Gaziantep Büyükşehir Belediyespor.

In August 2011, Adu returned to Major League Soccer, signing with Philadelphia Union, reuniting him with former D.C. United and United States national U-23 coach Piotr Nowak. Though terms were not disclosed, as per club policy, it was stated that he was not a designated player. Adu debuted for Philadelphia Union on August 13 in a match against FC Dallas and scored his first goal for the club on September 7, 2011 against New England Revolution.

On April 5, 2013, Adu joined Bahia of Brazil. The deal also saw Kléberson move in the opposite direction. He was released by the club on November 7, 2013.

After his release from Bahia he started a succession of unsuccessful trials around Europe. In February 2014, Adu began training with Blackpool. However, at the end of February they decided against offering Adu a contract, though they gave permission for him to continue training with club to aid his fitness. In June 2014, Adu began training with Norwegian club Stabæk, managed by former United States national team coach Bob Bradley. In July 2014, Adu joined Dutch club AZ Alkmaar on trial, but he did not convince the club to offer him a contract.

After a trial with AZ that ended without a contract, Adu signed with Serbian club FK Jagodina on July 24, 2014. On July 25, it was announced that Adu had signed a one-and-a-half year contract with Jagodina. Adu made his debut for Jagodina on September 24, 2014, coming on as a second-half substitute in a Serbian Cup match against FK BSK Borča. This was to be his only first-team appearance for the club. Adu was released by Jagodina, who opted not to renew his six-month contract, on December 21, 2014.

Following Adu’s release from Jagodina, Adu was linked with a move to the Swedish Allsvenskan in February 2015.

On March 28, 2015, KuPS announced the signing of Adu on a one-year contract, subject to passing a medical. Adu also played some games for KuPS’ reserve team SC Kufu in the Finnish third division. On July 7 of the same year Adu exercised an option in his contract to terminate it.

On July 14, 2015, Tampa Bay Rowdies of the North American Soccer League announced the signing of Adu. Contract details of the signing were not disclosed. It was his 12th professional club. Adu made 13 appearances for the club before being released after the 2016 NASL season.

Prior to the 2017 MLS season, Adu went on trial with the Portland Timbers but was not offered a contract. Adu then went on trial with newly promoted Polish Ekstraklasa side Sandecja Nowy Sącz in August 2017. Adu joined the expansion Las Vegas Lights FC on a trial basis prior to the 2018 season. Adu was not selected for either of the team’s first two preseason matches. He was selected for the third and final preseason match against his former club D.C. United, playing 30 minutes and notching an assist—the first assist recorded in club history and his first in nearly three years—in a 4–2 defeat.

On March 15, 2018, Las Vegas added Adu to the regular season roster. Adu made 14 appearances for the Lights and scored 1 goal for the club during the 2018 campaign. Despite this, Adu was released by Las Vegas at the end of their 2018 season.

On October 14, 2020 after a two-year hiatus, in which Adu worked as a youth football coach in Maryland, it was announced that Adu had joined Österlen FF, recently promoted to the third tier of Swedish football (Division 1). After signing with the club, Adu expressed his excitement to play professional soccer again, noting that he “[s]kipped a lot of steps in the past but now [he] get[s] a chance to do it right.”