Castelletti has been city councillor of Brescia since 1991, when she was first elected as a member of the Italian Socialist Party.

[1] In 2008 she decided to run as an independent for the office of Mayor of Brescia, obtaining the 6% of the popular vote.

After receiving again the 6% of the popular vote, on the second round she agreed to endorse the centre-left candidate Emilio Del Bono, binding her support to her appointment as deputy Mayor in case of victory.

[3] On 31 March 2023 she became acting Mayor once Emilio Del Bono left the office after being elected regional councillor.

[4] In 2023 she was candidate again for the office of Mayor of Brescia, this time with the support of the centre-left coalition.