California's legalisation of recreational marijuana took effect at the beginning of January, making the state the largest legal market for pot in the world.
The move will affect thousands whose marijuana convictions hurt their employment prospects and obtaining certain government benefits, according to 'The San Francisco Chronicle'.
"While drug policy on the federal level is going backwards, San Francisco is once again taking the lead to undo the damage that this country's disastrous, failed drug war has had on our nation and on communities of color in particular," said District Attorney George Gascon.
"Long ago we lost our ability to distinguish the dangerous from the nuisance, and it has broken our pocket books, the fabric of our communities, and we are no safer for it.
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"A criminal conviction can be a barrier to employment, housing and other benefits, so instead of waiting for the community to take action, we're taking action for the community," he stressed.
The DA's office will review 4,940 felony convictions and dismiss and seal 3,038 misdemeanors.
Eight US states including Colorado and the federal capital Washington have already legalised recreational marijuana use.
Thirty US states and Washington DC have legalised medical marijuana use.
Some states, however, still believe it is a "gateway" drug toward other, harder illegal drugs, such as heroin or cocaine.
