Thor has been shown flying without Mjolnir in the comics but the creators aren't very consistent when it comes to his powers. What's clear, on the other hand, is that Thor can jump and leap very well which can make him look like he's flying.

Can Thor fly on his own?

Unlike Superman, who could fly just by raising his arms, Lee decided that Thor needed to spin his hammer incredibly fast and hurl it into the air--allowing him to be pulled into flight via an unbreakable strap attached to Mjolnir's handle. ...

Can unworthy Thor fly?

In other media its pretty clear he can only fly via twirling/throwing/controlling his hammer. Originally he was shown only to fly through the same way as well. ... Now recently (since being Unworthy) hes been shown A LOT that he needs his hammer for flight.