State Sen. Scott Wiener, D-San Francisco, said there is historical meaning in San Francisco taking this step.
There are stipulations in how Gascón's office will reclassify felony convictions to misdemeanors.
That, Gascon and others in San Francisco said, is part of the reason he is automatically changing or reviewing convictions. California law allows those with minor marijuana records to request dismissals, but it's a lengthy process.
Convictions make it hard for people to get jobs and obtain government benefits such as housing.
The National Conference of State Legislatures said nine states have marijuana laws that let people clear or modify their records.
The process will start immediately with no court hearings or appearances and will all done proactively at the DA's office.More news: Tottenham legend has advice for Mauricio Pochettino regarding Lucas Moura
The San Francisco District Attorney's office will reportedly reevaluate cases dating all the way back to 1975.
California's measure applied retroactively to hundreds of thousands of cases, but people who wanted to get their convictions erased or reduced had to petition the courts themselves. In 2016, voters chose to legalize medical marijuana.
Assemblyman Rob Bonta, a Democrat from Oakland, introduced legislation on January 9 that would require county courts to automatically expunge eligible records.
"The #WarOnDrugs was a failure, it's time we take action to undo the damage it has done", Gascon said.
"If you've made a legislative determination that this is no longer criminal, why would you want to continue to have people feeling the ramifications of something that people going forward will no longer have to suffer?" said Jenny Roberts, an American University law professor.
In order to "provide a clean slate for many Californians", Gascon said the district attorney's office will not only expunge conviction records, but will also expunge arrest records for those who are eligible.
Possession of 8 grams or less of Concentrated Cannabis pursuant to Health and Safety Code Section 11357, when he/she was 21 years or older, may have their record of conviction dismissed.