Nicholls has signed a four-year deal at Leeds, tying him to the club until 2010. He is likely to make his debut in Saturday's friendly clash at Nottingham Forest.
He began his playing career at Charlton Athletic making his debut in the 1996-97 season.
After an injury ruled him out most of the 1997-98 season he failed to break into the team the following season and was loaned to Brighton & Hove Albion for four games.
After a short period back at Charlton he was transferred to Wigan Athletic for £250k (rising to £600k based on appearances).
In the 1999-2000 season he was once again victim of injury, only playing 8 games and after he appeared in 26 games in the 2000-01 season Joe Kinnear signed him for Luton for £150k.
Nicholls was a pivotal player in Luton's promotion season of 2001-02 becoming the team captain, he had also been captain of the England under 18s, under 19s and under 20s teams.
At Luton, Nicholls managed for several seasons to avoid the injuries that dogged him at previous clubs and became the "heart and soul of the team" according to Kinnear.
Unfortunately the 2003-04 season was not so good as he was again injured and missed 5 months in addition to falling out with new Luton boss Mike Newell over his playing position.
After the injury he returned to his preferred central midfield position and contributed to Luton's very respectable 10th place finish. Nicholls is a traditional hard man who picks up more than his share of cards, he earned a yellow in his Luton debut and two more in the next game and has been a regular name in referee's notebooks ever since.
At 5'10" and 11st he uses his size to good advantage but can also play the ball and has been Luton's penalty taker, corner taker and dead ball expert at various times.
In the 2004-2005 season he was voted Player of the Year by the supporters as Luton won the League One championship.
It was he who lifted the championship trophy following their 4-2 victory against Brentford. He scored 14 goals, most of which came, crucially at times, from the penalty spot.