Liverpool-born Greenwood joined Everton at the age of eight, and progressed through the club's Centre of Excellence. In August 2010 she made her first team debut in the 2010–11 UEFA Women's Champions League qualification round.
When Everton's longstanding left-back Rachel Unitt signed for Birmingham City after the 2011 FA WSL season, Greenwood took over her place in the team. Throughout the 2012 season she performed so well that she was named FA Young Player of the Year at FA Women's Football Awards in November.
Greenwood played in Everton's 2–0 2014 FA Women's Cup final defeat by Arsenal. When the 2014 season culminated in Everton's relegation, Greenwood requested to leave in order to protect her national team place ahead of the 2015 FIFA Women's World Cup. She signed for Notts County on a two-year contract for an undisclosed transfer fee. The move disappointed Everton who wanted Greenwood to leave on loan and come back if they were promoted again.
In January 2016, she joined Liverpool.
Greenwood captained England at youth level and played at the 2012 UEFA Women's Under-19 Championship. Senior national team coach Mark Sampson selected her for the 2014 Cyprus Cup, where she made her debut against Italy on 5 March 2014. She scored her first goal for England in September 2014, during a 10–0 win in Montenegro.