Born in Vienna, Alaba began his career with SV Aspern, his local club in Aspern, in the 22nd district, Donaustadt, before joining the youth setup of FK Austria Wien at age 10. He rose through the ranks quickly, and in April 2008 he was named on the first-team substitutes’ bench for a Bundesliga match. He also played five times for Austria Wien’s reserve team, before leaving in summer 2008 to join German Bundesliga side Bayern Munich.

He started out in Bayern’s youth system, playing for the under 17 and under 19 teams, before being promoted to the reserve team for the 2009–10 season. He made his debut in a 3. Liga match against Dynamo Dresden in August 2009 and scored his first professional goal for FC Bayern Munich II on 29 August 2009. He was named in Bayern Munich’s squad for the 2009–10 UEFA Champions League, where he was assigned the shirt number 27. In January 2010, it was announced that Alaba would train with the first team for the rest of the 2009–10 season, along with reserve teammates Diego Contento and Mehmet Ekici.

Alaba played for Austria at under-17, under-19 and under-21 level. In October 2009, he was called up to the senior Austria national team for a match against France. He made his debut in this game, making him the youngest player in the history of the Austrian national team. He scored his first goal for Austria on 16 October 2012 in a World Cup Qualifying game at home to Kazakhstan where Austria were 4–0 winners.

The trio were named on the first-team substitutes’ bench for the first time on 10 February 2010, for a DFB-Pokal game against SpVgg Greuther Fürth, and Alaba came on in the 59th minute, replacing Christian Lell. After one minute on the pitch, and with his second touch of the game, he set up Franck Ribéry to give Bayern a 3–2 lead, in a match they went on to win 6–2. He also became Bayern’s youngest ever player in a competitive fixture, at 17 years, 7 months and 8 days old. At age 17 years, 8 months and 13 days, he made his UEFA Champions League debut against ACF Fiorentina on 9 March 2010.

In January 2011, Alaba joined TSG 1899 Hoffenheim on loan until the end of the 2010–11 season. Later that month he scored his first-ever goal in the Bundesliga in a 2–2 draw with FC St. Pauli.

Alaba returned to Bayern at the start of the 2011–12 season, where he became a regular member of the first-team squad. On 23 October 2011, Alaba scored his first league goal for Bayern in the 1–2 away loss against Hannover 96. On 20 December 2011, Alaba was named Austrian Footballer of the Year for the first time. During the second half of the 2011–12 Bundesliga season, he established himself as a starter for Bayern. On 25 April 2012, he played in the 2011–12 UEFA Champions League semi-final second leg against Real Madrid, and scored Bayern’s first kick of the shootout as they won 3–1 on penalties. However, due to being booked in the semi-final, Alaba was unable to play in the 2012 UEFA Champions League Final through suspension.

At the age of just 19, Alaba won the prestigious vote for Austrian Footballer of the Year in 2011. In the poll organised by APA (Austrian press agency) among the coaches of the ten Austrian Bundesliga clubs, Alaba finished top on 21 points, just ahead of Austria Wien’s Dutch star Nacer Barazite (20 points) and VfB Stuttgart’s Martin Harnik. In December 2012, he received the award for the second consecutive year. On 10 September 2013, Alaba scored the only goal of the game against Ireland in the 84 minute to give Austria a 1–0. He scored the final goal in Austria’s 3–0 win in the final qualification match against Faroe Islands.

On 5 December 2012, Alaba scored his first Champions League goal in a 4–1 win over BATE Borisov in the 2012–13 season. On 18 December 2012, Alaba was named Austrian Footballer of the Year for the second year running.

On 2 April 2013, Alaba scored the seventh-fastest goal (25.02 seconds) in Champions League history to set Bayern on their way to a 2–0 win over Juventus. On 25 May 2013, he played the full 90 minutes at left-back as Bayern beat Borussia Dortmund 2–1 in the 2013 UEFA Champions League Final.

During the 2013–14 season, he played in the German Super Cup against Borussia Dortmund, the UEFA Super Cup against Chelsea, and two matches in the FIFA Club World Cup; the first against Guangzhou Evergrande and the second against Raja Casablanca. On 2 December 2013, Alaba signed a new contract with Bayern Munich, which expires in 2018. On 20 December, Alaba was named Austrian Footballer of the Year for the third year running.

On 18 December 2014, Alaba was named Austrian Footballer of the Year for the fourth year running. In January 2015, Alaba was voted as the left-back in the UEFA.com Team of the Year 2014 by users (352,070 votes) for the second consecutive year. On 31 March 2015, Alaba suffered a medial ligament damage in his left knee during Austria’s 1–1 friendly draw with Bosnia and Herzegovina. Alaba missed the rest of the season for Bayern because of the injury.

He made his 2015–16 season debut in the German Super Cup on 1 August, a penalty shootout defeat to VfL Wolfsburg. Three weeks later against his former club 1899 Hoffenheim, Alaba’s misplaced pass resulted in Kevin Volland scoring in nine seconds, the quickest Bundesliga goal of all time. On 17 December 2015, Alaba was named Austrian Footballer of the Year for the fifth consecutive year.

He scored a ninth-minute Panenka-style penalty kick on 8 September 2015 to open a 4–1 away win over Sweden at the Friends Arena in Solna. The result qualified Austria for UEFA Euro 2016, their first successful qualification campaign for the continental championship. Alaba played in all three group matches as Austria finished last in their group with one point.

On 8 January 2016, Alaba was voted as the left-back in the UEFA.com Team of the Year 2015 for the third consecutive year. On 18 March 2016, Alaba extended his contract until 2021. On 21 December 2016, Alaba was named Austrian Footballer of the Year for the sixth year running.

On 24 March 2017, he captained Austria for the first time in a World Cup 2018 Qualification game against Moldova in a 2–0 victory, deputizing in the absence of suspended captain Julian Baumgartlinger. Alaba played in eight matches as Austria failed to qualify for the 2018 FIFA World Cup.

On 10 February 2018, Alaba made his 200th Bundesliga appearance in a 2–1 win over Schalke.

During the 2019–20 season, Alaba primarily played in the Centre Back position, often partnering up with Jérôme Boateng. He adapted to the position very well and soon became the leader of the defense. On 23 August 2020, he played the full 90 minutes at centre-back as Bayern beat Paris Saint-Germain 1–0 in the 2020 UEFA Champions League Final. Bayern Munich went on to have their second remarkable treble-winning season after 2012–13, and thereby Alaba securing his second continental treble in his career.