Most famously Richard Nixon received a pardon by Ford immediately after his resignation but before any prosecution. Also, it’s any federal law, the exception is impeachment and nothing else.

So, pardons can very much apply before conviction or even prosecution. They may not pardon someone for something that hasn’t happened, but as long as there in office when the crime is committed that’s more a technical issue.

https://en.wikipedia.org/wiki/Burdick_v._United_States

After President Gerald Ford left the White House in 1977, close friends said that the President privately justified his pardon of Richard Nixon by carrying in his wallet a portion of the text of the Burdick decision, which stated that a pardon carries an imputation of guilt and that acceptance carries a confession of guilt.[6] Ford made reference to the Burdick decision in his post-pardon written statement furnished to the Judiciary Committee of the United States House of Representatives on October 17, 1974.[7] However, the reference related only to the portion of Burdick that supported the proposition that the Constitution does not limit the pardon power to cases of convicted offenders or even indicted offenders.[7][8]