Bec dated Australian actor Beau Brady for four years (2000-2004) prior to beginning her relationship with tennis star Lleyton Hewitt. After 6 weeks of dating, Lleyton proposed to Bec and on 5 May 2005, the pair announced that Bec was pregnant.

Hewitt and Cartwright married on 21 July 2005. The wedding was at the Sydney Opera House and their reception took place at Taronga Zoo, and was given much media attention. Fellow actors and friends Ada Nicodemou and Kate Ritchie were bridesmaids for Bec.

On 29 November 2005, she gave birth to daughter, Mia Rebecca Hewitt, at Sydney's North Shore Private Hospital. Mother and daughter now travel with Lleyton as he plays tennis all over the world.

In June 2008 it was announced that the couple are expecting their second child in January 2009, during the time of the Australian Open, after much speculation.