A lifelong supporter of Newell's Old Boys, Messi joined the Rosario club when he was seven years old. During the six years he played for Newell's, he scored almost 500 goals as a member of "The Machine of '87", the near-unbeatable youth side named for the year of their birth, and regularly entertained crowds by performing ball tricks during half-time of the first team's home games. His goalscoring idol growing up was Brazilian striker Ronaldo, with Messi calling him "the best forward I've ever seen".

However, his future as a professional player was threatened when, aged 10, he was diagnosed with a growth hormone deficiency. As his father's health insurance covered only two years of growth hormone therapy, which cost at least $1,000 per month, Newell's agreed to contribute, but later reneged on their promise. He was scouted by Buenos Aires club River Plate, whose playmaker Pablo Aimar he idolised. It was speculated that he failed to sign with River Plate due to his ill health; however, in a 2019 interview, Messi revealed that River Plate had offered to pay for his medical treatment after he scored four goals at a trial. River wanted to sign him, but Newell's Old Boys, who had refused to pay for his treatment, also refused to release his player card, preventing the transfer.
As the Messi family had relatives in Catalonia, they sought to arrange a trial with Barcelona in September 2000. First-team director Charly Rexach immediately wanted to sign him, but the board of directors hesitated; at the time it was highly unusual for European clubs to sign foreign players of such a young age. On 14 December, an ultimatum was issued for Barcelona to prove their commitment, and Rexach, with no other paper at hand, offered a contract on a paper napkin. In February 2001, the family relocated to Barcelona, where they moved into an apartment near the club's stadium, Camp Nou. During his first year in Spain, Messi rarely played with the Infantiles due to a transfer conflict with Newell's; as a foreigner, he could only be fielded in friendlies and the Catalan league. Without football, he struggled to integrate into the team; already reserved by nature, he was so quiet that his teammates initially believed he was mute. At home, he suffered from homesickness after his mother moved back to Rosario with his brothers and little sister, María Sol, while he stayed in Barcelona with his father.

After a year at Barcelona's youth academy, La Masia, Messi was finally enrolled in the Royal Spanish Football Federation (RFEF) in February 2002. Now playing in all competitions, he befriended his teammates, among whom were Cesc Fàbregas and Gerard Piqué. After completing his growth hormone therapy aged 14, Messi became an integral part of the "Baby Dream Team", Barcelona's greatest-ever youth side. During his first full season (2002–03), he was top scorer with 36 goals in 30 ga…

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Messi began the 2004–05 season as a guaranteed starter for the Barcelona B team, but after some lobbying by the senior players, he was promoted to the first team by manager Frank Rijkaard. He made his La Liga debut for Barcelona on 16 October 2004 against Espanyol, and scored his first senior goal on 1 May 2005 against Albacete, from an assist by Ronaldinho, becoming at that time the youngest-ever scorer for the club. At 17 years, three months, and 22 days old, he was at the time the youngest player to represent Barcelona in an official competition, and the club won the league title during that season.

On his 18th birthday, he signed his first contract as a senior team player, which was updated three months later to keep him at the club until 2014. Barcelona began the 2005–06 season by winning the Supercopa de España without Messi, who was not selected to participate in the competition. By the end of the season, the team won La Liga again as well as the UEFA Champions League, although Messi did not play in the final for the latter due to injury.

During the 2006–07 season, Messi scored his first hat-trick in a Clásico against Real Madrid, the first player to do so in 12 years. An incredibly finesse goal from Messi against Getafe and another goal scored by a handball against Espanyol gained notice for their similarities to the two famous goals scored by fellow Argentine Diego Maradona in the 1986 World Cup match against England, drawing comparisons between the two that Messi would face throughout his career. The team finished the season with only one trophy – the 2006 Supercopa de España. Barcelona finished the next 2007–08 season trophyless, leading to Rijkaard's departure.
At the beginning of the 2008–09 season, his first under Barcelona's new manager, former captain Pep Guardiola, Messi was given the number 10 shirt. Over time, he effectively became the tactical focal point of Guardiola's possession-based system, increasing his goalscoring rate as a result. During that season, Messi scored 38 total goals, and alongside Samuel Eto'o and Thierry Henry, contributed to a total of 100 goals in all competitions, a record at the time for the club. Messi played as a false nine for the first time in a Clásico against Real Madrid, setting up his side's first goal and scoring twice in the team's greatest-ever score at Santiago Bernabéu Stadium. He played his first final, winning the Copa del Rey. In addition, Barcelona won the La Liga title and later won the Champions League, thus achieving the first treble in the history of Spanish football.

During the first half of the 2009–10 season, Barcelona would also win the Supercopa de España, UEFA Super Cup and FIFA Club World Cup, resulting in becoming the first club to achiev…

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As a dual Argentine-Spanish national, Messi was eligible to play for the national team of both countries. He debuted for Argentina in 2004 for Argentina's U20 team against Paraguay, and was subsequently included in the squad for the 2005 South American U-20 Championship, where they would finish third. Messi would then lead the team to victory in the 2005 FIFA World Youth Championship, his first success with Argentina. Finishing the tournament with six goals and two assists, he would also win the Golden Ball.

Messi would make his debut with the senior national team in 2005, at age 18, coming off the bench in a friendly against Hungary. Messi would have his first start in 2006 against Peru, and would score his first international goal in a friendly against Croatia. His World Cup debut came in the 2006 FIFA World Cup as a substitute in the 74th minute against Serbia and Montenegro, assisting one goal and scoring the final one in their victory. This made him the youngest player to represent and score for Argentina in the World Cup. He would represent Argentina again for the 2007 Copa América, where they would ultimately lose in the final. Messi was named the best young player of the tournament, having scored two goals and provided one assist. The 2008 Summer Olympics would mark another major achievement with his country, as he led Argentina's U23 team to claim the Olympic gold medal over Nigeria. Messi registered two goals and three assists throughout, and was singled out by FIFA as the stand-out player from the tournament's best team.

With the international retirement of Juan Román Riquelme, Messi was given Argentina's number 10 shirt. During a 2010 FIFA World Cup group stage match against Greece, where a majority of the starters rested due to a secured place in the knockout rounds, Messi would wear the captain's armband for the first time. Argentina were ultimately eliminated in the quarterfinals against Germany during that tournament, but Messi was identified as one of the tournament's 10 best players due to his pace and creativity, despite failing to register a single goal and only having one assist. The appointment of Sergio Batista ahead of the 2011 Copa America resulted in Argentina now building their team around Messi. However, Messi would again be goalless during the tournament but had three assists. The team would ultimately lose to Uruguay in penalties during the quarter-finals.

Following their unsuccessful performance, Batista was replaced by Alejandro Sabella, who awarded 24-year-old Messi the captaincy of the squad. The next several years saw many frustrations for Messi due to his inability to captain Argentina to an international trophy. During the 2014 FIFA World Cup, Mes…

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