Ellis was elected to the House of Representatives for the Division of Adelaide, South Australia at the 2004 federal election, defeating Liberal Party incumbent Trish Worth on a 2 percent two-party swing to a margin of 1.3 points, increasing to 8.5 points in 2007. The margin was reduced to 7.7 points in 2010 and to 4.0 percent in 2013.

After the 2007 election Ellis became Minister for Youth and Minister for Sport. This made her the youngest person to become an Australian government minister, a record until then held by former Prime Minister Paul Keating. In 2009 Ellis took on the portfolio of Early Childhood Education, Childcare and Youth. After the 2010 election, Ellis became the Minister for Employment Participation and Early Childhood and Childcare and the Minister for the Status of Women. For a few months prior to the 2013 Labor government defeat, Ellis was the Minister for Early Childhood, Childcare and Youth. Ellis was then elevated to shadow cabinet with the portfolios of Education and Early Childhood.

Ellis’s public image has been the subject of media attention since her entry into public life. In October 2008, she was voted Parliament’s “sexiest” MP in a poll of federal MPs conducted by The Courier-Mail newspaper. In April 2010, she modelled a Karen Millen dress and Gucci high-heels for Grazia magazine. She agreed to do the shoot to raise awareness of poor body image and in order to encourage fashion magazines to promote healthy attitudes toward weight and eating.

In 2012 Ellis voted in support of a same-sex marriage bill. She supported the Safe School Coalition Australia program in 2016.

Ellis is married to News Limited journalist David Penberthy, a former editor of Sydney’s Daily Telegraph and current editor of Adelaide’s Sunday Mail. They were married on 23 February 2013. Ellis gave birth to her first child in April 2015 and a second in July 2017. Ellis supports the Adelaide Football Club and was named as a club ambassador in 2009. In 2011, she joined tennis star Lleyton Hewitt as the club’s number-one ticket holder, becoming the first woman so honoured. She is also a Club Ambassador for the SANFL team the Glenelg Tigers.

On 9 March 2017, Ellis announced that she would step down from shadow cabinet as of the next reshuffle and leave parliament at the 2019 federal election in order to spend more time with her young family. A week later it was announced that she was in the early stages of her second pregnancy. Their second son was born in July 2017.