It generally is public information in which elections a voter has voted.

You cannot see how they voted, but you can see that they voted.

This is how all political campaigns know which people always vote, which people usually vote, which people sometimes vote, and which people never vote.

For example, in Washington State there is a voter registration database. These are public records that can be requested from the state (https://www.sos.wa.gov/elections/vrdb/default.aspx). In them, the date that the person last voted is listed (see "lastVoted" in column 36 https://www.sos.wa.gov/_assets/elections/vrdbdatabasefields....).

In addition, King County ballots come with a unique identifying code that you can look up on their website to see if your ballot was received or counted (https://info.kingcounty.gov/kcelections/vote/myvoterinfo.asp...).

It's actually even more than that. The list of people who have voted is available for public inspection on election day itself. In close races, campaigns will send people out to review the list(s), note which of their likely voters haven't voted yet, then call them to offer a ride or otherwise help them get to the polls so they can vote. It's not that common at the presidential level, but for city council or the like, it can make a real difference.

Do you have a source for this? I tried to find the laws on this but don't really see anything that discusses accessing a list of who voted during an election. I feel this this should be inaccessible until after the election.

Looks like it depends on the state: https://www.rcfp.org/open-government-sections/i-election-rec...

Most common is allowing the inspection of the list of absentee requestors, like in North Carolina: The chief election judge of each precinct is required to post one copy of the precinct absentee ballot list “in a conspicuous location in the voting place.”

That said, there are other states that do not allow this, so it's less widespread than I originally thought, and when it is allowed, it's usually either just the absentee requestors, or the list is made available after the election, not during. Sorry.