Gerrans was born in Melbourne, Victoria and grew up in Mansfield, Victoria.
He took up cycling after injuring his knee and speaking with his neighbour, former Yellow Jersey holder Phil Anderson whom he credits with introducing him to the sport. Gerrans was an Australian Institute of Sport scholarship holder
In 2002, he finished fifth in the senior Australian National Road Race Championships, and took the under 23 title. He went on to ride as a trainee with the Carvalhelhos-Boavista team, based in Portugal from 1 September 2003, and then as a trainee for the AG2R Prévoyance team from 1 September 2004. He turned professional in 2005, staying with AG2R Prévoyance, and participated in his first Tour de France in the same year.
Gerrans underwent surgery at a hospital in Nice following a heavy fall in the GP d’Ouverture la Marseillaise in February, 2006. A pin was inserted into his shattered left collarbone and a screw put into his broken right shoulder, and had stitches in his head.6 He resumed training three weeks later and went on to represent Australia at the 2006 Commonwealth Games in Melbourne.
In 2008, Gerrans rode for the Crédit Agricole team. He won stage 15 of the Tour de France, the high point of his career so far, after being in the four man breakaway for most of the day. Barely surviving attacks from the other strong climbers in the breakaway, in which the fourth rider was dropped from the group, he eventually sprinted away in the last few dozen metres, without a response from the two remaining contenders.
Following the closure of the Crédit Agricole team Gerrans signed with the UCI Professional Continental Cervélo TestTeam for the 2009 season. Despite his success of the previous year, he was not included in the squad for the 2009 Tour de France.
On stage 14 of the 2009 Giro d’Italia Gerrans attacked his breakaway companions on the short steep climb of San Luca, near Bologna, to win the stage – the first Grand Tour stage victory for Cervélo TestTeam. After winning 10th stage of the 2009 Vuelta a España Gerrans became the first Australian to win a stage of each of the three Grand Tours.
He signed with Team Sky for season 2010 and made the Team Sky selection for the 2010 Tour de France. Gerrans was involved in a large crash on Stage 8 of the race resulting in a broken arm and his withdrawal from the race.
In 2011, Gerrans came 3rd in the Amstel Gold Race. In August, he won the Danmark Rundt. Shortly after that victory, it was announced that Gerrans would join GreenEDGE for the team’s inaugural season in 2012. In January 2012, Gerrans became national road race champion for the first time, out-sprinting Lampre-ISD’s Matthew Lloyd and Team Sky’s Richie Porte for victory. Later in the month he won the Tour Down Under for the second time. On 17 March 2012, Gerrans won Milan – San Remo in a three-man sprint finish, beating RadioShack-Nissan-Trek’s Fabian Cancellara and Liquigas-Cannondale’s Vincenzo Nibali to the line in Sanremo.