Sully began at the Seven Network in Brisbane and moved to Network Ten in 1991 where she began working as a political correspondent in the network's Canberra bureau. She moved to Ten Sydney as a reporter, update presenter and later as a weekend late-night presenter for Ten Eyewitness News. Her big break came in 1992 when she became co-presenter on Good Morning Australia with Mike Hammond and later Ron Wilson. The following year the pair moved from dawn to dusk, presenting Ten Sydney's 5:00pm news. In November 1995, following Anne Fulwood's departure for theSeven Network, Sully became the regular weeknight presenter of Ten's Late News, a position she held until 4 November 2010.
Sully was the first Australian television journalist to cover news of the September 11 terrorist attacks. She was on air when the first attack occurred and, shortly afterwards, began presenting live breaking coverage. She was also the first television reporter to broadcast from the site of the 1997 Thredbo landslide disaster in which 17 people were killed. She covered the rescue efforts until the sole survivor, Stuart Diver, was rescued in dramatic fashion.
More recently Sully travelled to Timor to cover how the new nation is coping post the uprising. Her story documents the struggle for independence post liberation.
In 2009 Sully travelled to Borneo to cover the tragic story of World War Two diggers who died at Sandakan and on the infamous Death Marches. The documentary premiered on Network Ten over the Anzac Day weekend that year.
During Sully's time with Network Ten she has become the face of Ten News and hosted various specials, including the anniversaries of the September 11 attacks and the 2002 Bali bombings as well as a science and nature documentary series that aired prime time for several months. She hosted the Australian of the Year Awards program, presented from Canberra on Australia Day eve, from 2002 to 2007. Through 2005 and 2006, she hosted various specials of Network Ten's Australia's Brainiest. She also regularly hosts the New Year's Eve special of Ten News, reviewing the major news events of the year. She has also informed the finalists of Big Brother Australia 2005, 2006, 2007 and 2008 of newsworthy events that occurred during their time in the "house" in the style of a Ten News broadcast. She also hosted Cool Aid Australia's National Carbon Test.
Sully is a keen sports fan  and covered the 1994 Commonwealth Games in Victoria, Canada and the 1996 Summer Olympic Games in Atlanta. She was the first woman to co-host Australia's iconic Melbourne Cup carnival and did so for seven years.
In 2006 a character based on Sully named "Sandra Sultry" featured in The Wedge, a comedy sketch show based around the fictional town of Wedgedale. "Sultry" was a news reporter who constantly revealed her legs, talked in a seductive, husky voice and chatted with other people in the studio while the cameras were running.
In September 2010, Sully was announced as presenter of Sydney's Ten Evening News which commenced in January 2011.
In April 2011, Sully was reinstated as Ten Late News presenter with Kathryn Robinson returning to Finance presenter on Ten Early News due to poor ratings for 6PM with George Negus and Ten Evening News.