Tremlett started career at Hursley Park CC. Tremlett took a wicket with his first ball in first-class cricket against New Zealand A in 1999, dismissing Mark Richardson. He went to India with the Under-19s in 2000/01, and was one of the first cricketers to attend the ECB Academy the following year.

Tremlett won the NBC Denis Compton Award in both 2000 and 2001. His grandfather Maurice played three times for England in the 1940s and also for Somerset, while his father Tim turned a playing career into a coaching job at Hampshire, and has coached his son during Chris's entire professional career.

In 2005 he made his England debut in a One Day International against Bangladesh at Trent Bridge, Nottingham, taking 4–32 and missing out on a hat-trick only by a freak occurrence: the hat-trick ball fell onto the top of Mohammad Ashraful's stumps, but did not dislodge a bail. To demonstrate his continuing progress Tremlett was regularly selected in England's 12 for the Ashes Tests later in 2005 but due to the success and consistency of the first choice bowlers he did not make his debut. He was also selected for the 2005–06 season tour to Pakistan, but had to withdraw because of injury. After undergoing surgery to his right knee and hip, he also missed the tour to India.

Tremlett was in and out of the Hampshire team in April and May, playing two Championship matches against Sussex and Warwickshire, and taking four wickets, and was then diagnosed with a side strain. He returned to take six wickets in seven matches in Hampshire's Twenty20 Cup campaign, and featured in four more Championship matches, to bring his tally for the season to 30 wickets at a bowling average of 21.70, the lowest at the club,