Brooks, who holds both German and American citizenship, participated in several camps for both the U.S. U-20 and U-23 team and in one for the Germany U20 team. He made his international debut for the U.S. Under-20 team in a 5–0 loss against Paraguay in September 2010.

Hertha suffered relegation from the Bundesliga in 2012. After the departure of some players and his form in preseason, Brooks made his professional debut as a starter in Hertha’s 2. Bundesliga opener on August 3, 2012, a 2–2 draw with SC Paderborn 07 at the Olympiastadion. He played 29 matches over the season as the team won the league title and promotion, and scored once in the last game of the season on May 19, 2013, a late equalizer in a 1–1 home draw with Energie Cottbus.

Brooks scored in his Bundesliga debut on August 10, 2013 in Hertha BSC’s 6–1 victory against Eintracht Frankfurt. His appearances over the season were limited by injury and poor form, and he also missed time in April 2014 while recovering from the application of a large back tattoo, a decision that did not please manager Jos Luhukay.

In July 2013, German magazine kicker indicated that Brooks had received a call-up from the U.S. senior national team to feature in a friendly against Bosnia and Herzegovina. Brooks made his senior international debut in the 4–3 victory over Bosnia. Brooks was named in the United States’s final 23-man roster for the 2014 FIFA World Cup. In the opening group match against Ghana, he came on as a halftime injury substitute for Matt Besler and scored the winning goal in the 2–1 victory, a header off Graham Zusi’s corner kick. He became the first American to score as a substitute at the World Cup.

On December 17, 2014, Brooks opened the scoring for Hertha in a 4–4 draw with Eintracht Frankfurt that saw Berlin throw away a two-goal lead in the final minute. During the 2014–15 Bundesliga season, Brooks won 68% of his direct duels in a total of 27 league appearances for the club, ranking third best among Bundesliga players making at least 17 appearances. Brooks also had a pass accuracy of 75% while finishing top 12 in the Bundesliga in clearances per match.

On June 5, 2015, Brooks scored in a friendly against the Netherlands in Amsterdam as the United States fought back from a 1–3 deficit to win 4–3, their first victory against the Dutch. Later that month, he was named in coach Jürgen Klinsmann’s squad for the 2015 CONCACAF Gold Cup, playing four matches as the team finished fourth.

Klinsmann also called Brooks up for the Copa América Centenario, the centenary edition of the Copa América held in the United States in 2016. He played all five matches as the nation lost 4–0 to Argentina in the semi-finals, but did not take part in the bronze medal match loss to Colombia.

On May 31, 2017, Brooks signed with fellow Bundesliga club VfL Wolfsburg. The reported €20 million transfer fee is the record highest for an American soccer player. He made his debut on August 13 in the first round of the DFB-Pokal away to fourth-tier Eintracht Norderstedt, but suffered a torn thigh tendon and was substituted in the first half of the 1–0 win, being ruled out for the next three months. On October 28, he returned to the team to start on his league debut for the club, a 1–1 draw at FC Schalke 04. Affected by injury all through his first season at the Volkswagen Arena, Brooks totaled just ten appearances before playing the full 180 minutes of the 4–1 playoff win over Holstein Kiel that kept his team in the Bundesliga for the following season.