Garnett made her international debut for England in 2000 against Spain. She continued to be selected for England throughout the decade, eventually earning her 100th cap in 2011 during the match against Canada. This made her the first women's rugby union international to earn 100 caps for England, and the third woman rugby player from any country to reach that mark. Garnett is also England's most selected hooker. She represented her country in three Women's Rugby World Cups, in 2002, 2006, and 2010. On each occasion, England reached the final, only to lose to New Zealand; Garnett was a member of the starting fifteen in each final.