Originally from Melbourne, Haynes made her WNCL debut for Victoria as a 19-year-old in in the 2005-06 season. She went on to become a key figure in the Spirit line-up and skippered her state to back-to-back WT20 titles in her first two seasons as captain. Haynes’ leadership potential was recognised early and in 2008 she was chosen to lead the Australian Youth team, guiding them to upset victories over the senior English and Australian teams.In 2009 she was selected in Australia's squad for their tour of England where she made 98 runs in her Test debut. She was dismissed in the final over of the first day in that match as the nervous nineties caught up with her! In 2011-12 Haynes moved north and joined the Lendlease Breakers. She had a major impact on the team that summer, especially in the WNCL Final. Haynes was standing at the other end as NSW lost two early wickets in the decider against Victoria. Along with Lisa Sthalekar, Haynes mounted a fight-back which saw her post 156 – the first ever century in a WNCL Finals match. Haynes was promoted to vice-captain of the Breakers and played a key role in the team winning both the WNCL and WT20 in the 2014-15 season.She has also established her place in the Australian team, playing in Australia’s World T20 and World Cup victories. She was Australia’s leading run scorer at 2013 the World Cup in India, with her form earning her place in the Team of the Tournament.
After three-and-a-half-years out of the Australian team, Haynes was recalled in February 2017 and subsequently named in the Australian Women's 2017 World Cup squad. Her remarkable comeback continued when she was named acting captain for the 2017-18 Ashes, filling in for injured Meg Lanning.
In 2018, Haynes was vice captain of her national side which went all the way to win their fourth ICC World T20 championship title, and again was the vice captain when Australia won an unforgettable T20 World Cup Final at the MCG in March 2020.
In 2020, Haynes captained the Sydney Thunder to the WBBL06 Championship.