Theo initially joined the Club as a scholar in January 2006 and signed a professional contract on his 17th birthday on March 16.
"I'm so pleased to be joining Arsenal, a club I have admired for a long time. Coming to Arsenal will give me the opportunity to work with world class players every day and play football at the highest possible level."
He became England's youngest ever player when he came on as a substitute against Hungary in May and, without playing a single game for Arsenal, was selected for the 2006 World Cup in Germany, though he didn't feature.
It wasn't until the opening day of the following season that Theo made his Arsenal debut, when he came on as a substitute in the first game at Emirates Stadium against Aston Villa.
Theo made his first start on October 14 against Watford in the Premier League though a nagging shoulder injury meant he was restricted to only 13 starts in all competitions. His first goal in an Arsenal shirt came on the big stage when he opened the scoring in the 2007 League Cup Final against Chelsea.
Arsène Wenger said Theo’s shoulder injury blighted the teenager’s campaign. "Until he had his shoulder injury he was very promising. After he had his shoulder injury he was 50 per cent of what he was before.”
The following season, Theo continued to grow into one of the country's brightest talents.
His next goal for Arsenal came in the Champions League against Slavia Prague on October 23 and he admitted it was nice to finally score in front of the Arsenal fans.
"It's a weight off my shoulders in a way. The fans have been brilliant to me since I joined the Club. Every game the cheers are massive. It was good to get those goals for them."
His first goals in the Premier League came on February 23 against Birmingham at St Andrews, in a game overshadowed by a horrific injury to Eduardo.
His campaign will be mostly remembered for the part he played in the Champions League Quarter Final against Liverpool at Anfield on April 8. Theo produced an amazing 60-yard run past four Liverpool players and set up Emmanuel Adebayor to score a goal that at the time seemed enough to send Arsenal through.
He made 20 starts in all competitions and 19 substitute appearances, scoring seven times throughout the season.
In June, the Club announced Theo would wear the No 14 jersey vacated by Thierry Henry.
He started the season as a regular on the right side of the Arsenal midfield and the teenager confirmed his quality with a stunning hat-trick as England beat Croatia 4-1 in Zagreb in September. "I am delighted of course," said Arsène Wenger. "I am not surprised because he is a player who has good timing with his runs and is a clinical finisher who can score goals."
Theo's progress suffered when he dislocated his right shoulder while on international duty with England in November, though he made a full recovery and returned to the Arsenal side as a substitute against Burnley on March 8. He made 27 starts during the season and scored six times.
In May, Theo signed a new long-term contract with the Club and said: “I’m absolutely delighted to have signed a new long term contract. I’m so happy here and just want to say thanks to everyone who has helped me get to this point in my career."
Theo's performances were always going to come under scrutiny in a World Cup year and the only regret is that Arsenal fans did not see more of him.
The 21-year-old's campaign was halted time and time again by injuries and, in reality, he never really got a proper 'pre-season' under his belt after sitting out most of last summer.
Theo had to wait until October to get his season underway because of a back problem picked up in August. But when he returned he did so in style, scoring against Blackburn Rovers. A knee injury then kept Theo out for a further three weeks and when he made his second comeback he struggled to find form immediately.
His 15 starts were supplemented by just as many substitute appearances and his ability to have a match-changing impact on games was still apparent, not least against Barcelona at Emirates Stadium.
Media scrutiny intensified as the World Cup drew closer and Theo showed flashes of his finest form, notably against Burnley when he caused no end of problems and scored a fine individual goal.