Januzaj was born in Brussels, Belgium, where his Kosovar-Albanian family had migrated in 1992 so his father, Abedin, could avoid being recruited by the Yugoslav People's Army. Januzaj's father was the eldest of six children from Istok, Kosovo. Januzaj's uncles, Januz and Shemsedin, were members of the Kosovo Liberation Army, which fought for Kosovar independence during the Kosovo War. Since Kosovo declared independence in 2008, Januzaj has regularly visited his extended family there during the summer.
Januzaj began his football career with FC Brussels but joined Anderlecht as a 10-year-old in 2005. He left Anderlecht for Manchester United at the age of 16 in March 2011, after impressing in a skills session in Brussels.
Towards the end of the 2012–13 season, United manager Sir Alex Ferguson promoted Januzaj to the first-team and he was registered with the number 44 jersey; he did not feature in the remainder of the season, but he was an unused substitute in the final game of the season away to West Bromwich Albion. For his performances during the season, Januzaj won the 2013 Denzil Haroun Reserve Team Player of the Year award. He was included in the first-team squad for the 2013–14 pre-season tour of Asia and scored in the final game of the tour against Hong Kong side Kitchee. He also started in Rio Ferdinand's testimonial; United lost 3–1 to Sevilla, but Januzaj provided the assist for United's only goal.
Januzaj made his competitive debut for Manchester United against Wigan Athletic on 11 August 2013 in the Community Shield, coming on for Robin van Persie with six minutes left to play. He made his Premier League debut a month later, coming on as a 68th-minute substitute for Ashley Young in a 2–0 home win over Crystal Palace on 14 September. On 5 October 2013, in what was his first start for the club, Januzaj scored twice as United came from behind to claim a 2–1 victory away to Sunderland. In an effort to stave off interest from other clubs, who could have signed Januzaj for "minimal compensation" on the expiry of his existing contract in June 2014, Manchester United signed Januzaj to a new five-year contract on 19 October 2013. On 3 December 2013, he was nominated for the BBC Young Sports Personality of the Year; at the time of his nomination, he had played in ten games for Manchester United.
Januzaj was left out of Manchester United's initial squad for the UEFA Champions League when it was registered in September 2013, which came as a surprise to some members of the media after his impressive start to the season. Manager David Moyes later explained this was a tactical decision to allow the club to register an additional player for the competition; as Januzaj had not been with the club long enough to be eligible for the 'List B' squad, which for 2013–14 included any player born after 1 January 1992 who had been eligible to play for the club for at least two years, he would have had to be registered as one of only 25 'List A' players, taking up a space that could have been used by another player. By waiting until 7 October 2013 to register Januzaj, he became eligible as a List B player, but this meant that he could not be selected for the club's first two matches in the competition, at home to Bayer Leverkusen and away to Shakhtar Donetsk. Despite being available from the third game of the tournament, at home to Real Sociedad, Januzaj was left out of both games against the Spanish club and the return match against Bayer Leverkusen. He made his Champions League debut at home to Shakhtar Donetsk on 10 December 2013.
Towards the end of the first half of the 2013–14 season, Januzaj gathered somewhat of a reputation for diving, beginning with a yellow card for a "clear dive" on his full debut against Sunderland in October 2013. He received another booking for simulation against West Ham United in December 2013, going to ground despite West Ham defender James Collins not offering a challenge. His third yellow card – which made him the joint second-most-booked player in the Premier League since sanctions for diving were introduced in 2008 – came against Tottenham Hotspur on 1 January 2014 and was the most contentious of the three; although Januzaj went to ground "easily" under a challenge from Tottenham defender Danny Rose, others are of the opinion that Rose "made no attempt to win the ball" and that Januzaj should not have been booked. David Moyes has admitted speaking to Januzaj about simulation, but he has repeatedly requested that match officials protect the winger from overzealous tackling by opposition defenders.
At the start of the 2014–15 season, Januzaj was allocated the number 11 shirt, previously worn by the recently retired Ryan Giggs. He made his first appearance of the season as a substitute in United's opening Premier League match; a 1–2 loss at home to Swansea City. His first start of the season came in a 2–2 draw with West Bromwich Albion on 20 October 2014, in which he struggled against physical defenders Sébastien Pocognoli and Joleon Lescott.
After not scoring in any competitions the previous season, and not making the bench for the first game of the new campaign, Januzaj scored his first goal for over a year on 14 August 2015, a close range 29th-minute strike which gave United victory at Aston Villa. His first appearance upon return from loan came against Southampton on 23 January 2016, when he replaced Cameron Borthwick-Jackson as a late substitute.
On 31 August 2015, Januzaj was loaned to German club Borussia Dortmund on a season-long deal. On 6 January 2016, he was recalled to United after only playing in 12 games for Dortmund in the first half of the season, starting three times.
On 12 August 2016, Januzaj agreed to join Sunderland on a season-long loan move from Manchester United, linking up with former Manchester United manager David Moyes. Januzaj made his debut the following day, coming off the bench against Manchester City in a 2–1 defeat on the opening weekend of the Premier League season. On his third game for Sunderland against Shrewsbury Town, Januzaj scored his first goal for the team, a late winner. On 24 September 2016, Januzaj suffered an ankle injury that ruled him out for at least six weeks during the 3–2 defeat to Crystal Palace.
Having failed to hold onto his place in the Manchester United squad, Januzaj left the club in July 2017, signing a five-year contract with Spanish club Real Sociedad.
In 2013, media rumored Januzaj was eligible to represent teams other than Belgium (through being born there and having a Belgian passport), Albania (through his Albanian descent) and Kosovo (non-FIFA member at that time and a partially recognized independence state). About this case, Abedin Januzaj (Adnan's father) revealed: "I am not closing the doors to Albania, because we (Adnan Januzaj's parents) are Albanian. However, they are closing the doors to themselves. I only deal with professionals on the issue." and Adnan would not represent Serbia, Turkey and Croatia because "we have no connections with these countries". In October 2013, also England manager Roy Hodgson stated that The Football Association was monitoring the player with a view to calling him up if Januzaj was naturalized as a British citizen (and passed the residency requirement of FIFA, according to David Moyes). However, despite widespread press coverage, Januzaj would not meet the requirements to play for England under the unique rules set out in the Home Nations agreement. The Home Nations agreement requires that players engage in a minimum of five years of education before the age of 18 within the territory of the relevant association and do not offer national team eligibility through residency.
According to FIFA eligibility rules, which mostly based on jus soli (eligible to represent place of birth), a limited jus sanguinis (eligible to represent parent and grandparent's POB) and naturalization by residency, despite Januzaj's maternal family had Turkish passports, they were not born in the territory of Turkey. For Albania, despite Januzaj was eligible to have an Albania passport through naturalization, it was uncertain that he had parental or maternal parent and grandparent born in the modern border of Albania, in order to make him eligible. Lastly, Januzaj did not have a British passport, nor a 5-year residency, thus even without Home Nations agreement, he is not eligible.
Despite having played competitively for Belgium at senior level, Januzaj remained eligible to represent Kosovo, as it was not a UEFA or FIFA member when he made his Belgian debut. However, once he made another appearance with Belgium after 2016, the year in which Kosovo became a member of FIFA, Januzaj would be locked into representing Belgium only.
In June 2013, the former coach of the Belgian under-18 and under-19 teams, Marc Van Geersom, declared that Januzaj had refused a call-up to the Belgian team a number of times, since he would prefer to play for Albania.
On 7 October 2013, former national team coach Marc Wilmots attempted to select Januzaj for the 2014 FIFA World Cup qualifying games against Croatia and Wales, but Januzaj refused the call-up, saying he still needs to make a choice. In response, Wilmots stated: "I wanted to send him a clear signal. Just as Zakaria Bakkali, Adnan is a very promising youngster and I would like to integrate him into the group. I do not want to force his hand. I respect his choice and consider discussing with Adnan and his family". On 16 October, discussing rumours that suggested Januzaj would choose to represent Belgium if he is included in the 2014 FIFA World Cup squad, Wilmots declared: "The boy was born in Belgium, should he say that he wants to play for Belgium – from there, there is no haggling. I'm not going to play that game. If you're at the top level in Manchester or elsewhere, show me your skills and if I need you I will call. I make no promises, to Januzaj or any person". The following day, Januzaj's father told Kosovar TV station KTV that he did not like the Belgian federation's approach: "I am not blackmailing them as I'm not asking for any national team to get my son. It is them who want Adnan. [...] you have to be professional in this issue. Belgium should follow the rules as well. I run his career in a sportive sense, and I have Belgium managers running the legal issues."
On 23 April 2014, Wilmots announced that Januzaj had committed himself to the Belgian national team. This message was confirmed the day itself by Januzaj's manager Dirk De Vriese. On 13 May 2014, Januzaj was included in Belgium's preliminary 24-man squad for the 2014 FIFA World Cup. He played a first game with the national team two weeks later in a match against Luxembourg, but he did not earn his first cap; the match was considered unofficial by FIFA on 4 June because Belgian coach Wilmots made 7 substitutions while only 6 are allowed in international friendlies. His official debut came in the next warm-up game, a 1–0 win over Tunisia on 7 June, replacing Eden Hazard after 73 minutes.
In 2014, New Balance announced that they were taking over Warrior Sports, and they would be intent on making their own football boots. They recruited many professional footballers, including Januzaj, who also featured in an advertisement promoting the new range.As of match played 17 September 2017
As of match played 16 November 2014
Manchester UnitedFA Community Shield: 2013
Denzil Haroun Reserve Player of the Year: 2012–13
Blue Stars/FIFA Youth Cup Golden Ball: 2013