As a child, he followed his father Don in being a Liverpool fan, and at the age of 10 took up playing the game with AFC Newbury. Theo scored 100 goals in 35 games for the club's under-11 team and in 2000 won the Peter Houseman League Cup after his side beat Down Grange 4-2 in the final. After impressing with the Berkshire outfit, Theo was offered a schoolboy contract to join Swindon Town's Centre of Excellence, but soon left to join Southampton.
In his first season at Southampton, Theo's career continued to progress. At 15 years and 175 days old, he became the Saints' youngest player to appear for their reserve team by coming off the bench against Watford. He also helped the youth team make the final of the FA Youth Cup, where they lost 3-2 on aggregate to Ipswich Town. Walcott's England career also took off after he broke into the Under-17 team where he scored five times in eleven appearances.
Season 2005/06 was Theo's breakthrough season. He signed a full-time professional contract and at just 16 years and 143 days old, he made a substitute appearance in Southampton's goalless draw at home against Wolves, which made him the youngest player ever to appear for the Saints. Two months later he not only made his first start, but he also scored in Southampton's 2-1 defeat at Leeds United. Subsequently, he then scored in his next two appearances at Millwall and at home against Stoke City. 2005 finished with Theo being named in the top three of the BBC's Young Sports Personality of the Year.
Following Theo's rapid rise to fame, Chelsea and Arsenal both made bids for the 16-year-old, however the Gunners were the ones to clinch the forward's signature as Arsenal signed him on January 20, 2006, paying an initial fee of £5m which could rise to a maximum of £12m.
Life at Highbury didn't immediately take off for Theo, as for the remainder of the 2005/06 season, he failed to make a first-team appearance in the red and white shirt of Arsenal, however, he did make the bench for Champions' League matches against Real Madrid and Juventus, as well as a Premiership match against Birmingham City. Walcott did though make appearances for the North London club's reserves. On his debut for the reserves, he scored in their 3-2 defeat by Portsmouth. He also went on to score against Charlton's reserves as well.
In February, 2006, Theo's England career continued to flourish after being promoted to the Under-19s. He scored on his debut in his side's 3-0 friendly win over Slovakia. Three months later, Walcott made a substitute appearance for the England B team against Hungary, where at 17 years and 75 days old, he became the youngest player to appear in a full England match.
The summer of 2006 was when Theo was a controversial choice by then England manager Sven-Goran Eriksson, after the Swede included the teenager in his preliminary squad for the World Cup in Germany. The England national team were struggling with injury doubts over strikers Wayne Rooney and Michael Owen, and with more established strikers available, like Jermain Defoe, James Beattie and Darren Bent, the nation questioned the logic of the decision. In the end, England went out on penalties to Portugal in the quarter-finals to Portugal. Theo failed to make an appearance.
Prior to season 2006/07, the striker/winger was expected to fight the likes of Thierry Henry, Emmanuel Adebayor, Robin van Persie and Jeremie Aliadiere for a place in the starting Arsenal line-up. However, Theo wasn't to be starring for the England national team for their friendly against Greece, as he was demoted to the Under-21s by new England manager Steve McClaren. He did though become the youngest to score for the England Under-21s with his goal against Moldova.
The season started brightly for Theo after he came off the bench at home against Aston Villa. The youngster provided the cross that eventually was put home by Gilberto Silva. It saw the Gunners draw their first match at the Emirates Stadium 1-1. Theo made another cameo appearance a few days later in the Champions' League home match against Dinamo Zagreb. He provided a low cross in the stoppage time for Matthieu Flamini to score the winner. The game ended 2-1 and saw Arsenal reach the group stages of the competition. Three days later away at Manchester City, Theo this time couldn't prevent his side losing 1-0.
Makes first full start for Arsenal on the 14th of October, when Arsenal beat Watford 3-0.
Sven however decided to include Walcott instead of proven Premier League goalscorers, to the disgutlment of some players.
Pundits and experts praised Walcott for his maturity at such a young age, and beleived he was now fulfilling his massive potential.