Tredwell was born in Ashford, Kent and was educated at Southlands Community Comprehensive School in New Romney. His father, John, played over 1,000 times for Folkestone F.C. and coached his son at Folkestone Cricket Club. Tredwell has continued to play cricket for Folkestone in the Kent Cricket League.

He made three appearances in youth Test matches for England Under-19 cricket team against the West Indies in August 2001. He made his debut in the first Test at Grace Road, Leicester, alongside future Test cricketers James Anderson and Matt Prior. He claimed six wickets during the series, at an average of 32.50.

He was offered a contract with Kent County Cricket Club in 2001 after playing for the youth team. He was one of 14 players who were selected for the ECB National Academy in Loughborough in 2003–04, and the subsequent tours of Malaysia and India.

He made his first List A appearance for Kent Cricket Board against Worcestershire Cricket Board in the first round of the 2000 NatWest Trophy, when he was run out for ten and took one wicket. He also appeared in the subsequent two rounds of the competition, until Hampshire knocked them out in the third round. He spent the remainder of the 2000 season playing for Kent Second XI. He once again appeared in the first three rounds of the renamed 2001 Cheltenham & Gloucester Trophy, scoring 71 opening against Buckinghamshire in the second round, and 57 from the same position in the third round defeat to Warwickshire. His first-class debut for Kent came later in the same season, against Leicestershire in July 2001, claiming the wicket of Aftab Habib as his first scalp in a match that Kent won by three wickets at Grace Road.

His one-day debut for Kent County Cricket Club came in the 2002 Benson & Hedges Cup, where in a rain-reduced match, he claimed the wicket of Owais Shah as he finished with one wicket for 26 runs. He made 19 List A appearances for Kent in 2002, finishing the season with 21 wickets at an average of 23.76.

He played just two County Championship games in 2005, but played extensively in the one-day game, earning an extension to his contract. He became more involved with the Championship side in 2007, scoring his maiden first-class century (116*) against Yorkshire, and returning then career-best bowling figures of 6/47 against Surrey.