Unitt started her footballing career with Wolves Women as a 14-year-old. She joined Everton Ladies for 2000–01, before signing a professional contract with Fulham L.F.C. the following season. After a successful spell at Fulham, she returned to Everton in 2004. Her achievements to date include winning the domestic treble with Fulham in the 2002–03 season and the 2010 FA Women's Cup with Everton.
In July 2005, Unitt joined American W-League side New Jersey Wildcats for the last seven games of their season, teaming up with England and former Fulham teammate Rachel Yankey. The Wildcats eventually won the W-League championship, beating the Ottawa Fury Women by 3–0 in the championship match with Unitt scoring the first goal. Unitt also trained with Malmö FF in December 2005, but, fearing burn-out, rejected a contract offer from the Swedish club.
At the end of the 2011 FA WSL season, Unitt put in a transfer request after eight years at Everton. Following negotiations a move to Birmingham City was announced in February 2012. After her time at the club was disrupted by injury, Unitt rejected Birmingham's offer of a contract extension in January 2014 and left the club as a free agent. She signed for Notts County on 30 January 2014.
Unitt first played for England in 2000, after 16 appearances for the U18s. Her senior debut came in August 2000, in a 1–0 friendly defeat to France, before 50,000 spectators at Stade Vélodrome. She scored her first international goal in England’s victory over the Netherlands in September 2004, and scored twice more in her home town of Walsall during England's 4–1 friendly win over the Czech Republic in May 2005.
Unitt was named the Nationwide International Player of the Year at the 2004 FA Women’s Football Awards. She won the same award in 2006, voted for by the fans on the FA website.