After serving six two-year terms on the council, he mounted a campaign for mayor in 2011 when incumbent Joseph C. O'Brien decided not to seek reelection due to family concerns.

[2][3] He defeated former mayor Konstantina Lukes, perennial candidate Bill Coleman, and Carmen Carmona by earning 48% of votes cast.

[5] As mayor, Petty has overseen the partial demolition and redevelopment of Worcester Common Outlets, an abandoned downtown mall.

This effort involves recreating a direct roadway between Union Station and Worcester City Hall and Common that was lost when the mall was built in the 1960s.

In early 2017, Petty apologized after he was caught on tape making disparaging remarks about people protesting a city council proposal.