The interaction between felony convictions and being able to vote varies by state. You can find a map of the rules at https://www.aclu.org/issues/voting-rights/voter-restoration/....

In Baltimore, Maryland, you cannot vote while in prison. But you can once you get out. Felony conviction or no.

(I honestly don't think that the population with felony convictions are significant factors in elections, but they can and do vote.)