Who (& when) was the first female executioner in the United States? I know that the execution of Rainey Bethea, the last person publicly executed, was to have been the first carried out by a woman, Sheriff Florence Thompson, but she backed out and had a deputy do it.
After 36 hours, I trust a WAG might be in order, namely that there hasn’t been one.
I’m not fully conversant with the US system but following the last public execution in 1936, I’m guessing there was no requirement for the identity of the executioner to be released to the general public. Maybe not even before then. Furthermore, such an occupation might be seen to carry a stigma in some social circles so not all state executioners, male or female, would have volunteered details of it to all and sundry.
Thee are exceptions of course. Joseph Francel served as New York State Executioner from 1939 to 1953. His name was no stranger to the newspapers of the time. In contrast, Francel’s successor Dow Hover took great pains to conceal his activities from prying eyes.
I would have thought that the existence of a female executioner in the US penal system would have emerged by now, simply because no matter how secret the information there is always somebody who knows, and who is prepared to tell.
Seems a little sketchy, but this site mentions the execution of Vincent Voychek by a “secretary in a courthouse office” named Zoe Himes in 1911.
I’m slightly sceptical about an article that records an execution taking place before the trial. 
Still, it remains a possibility. I’ve also found one source suggesting that a woman pulled the switch on Ted Bundy, but I’m not enthusiastic about its reliability.
As a side note, and widening the geography, allow me to introduce Lady Betty of Roscommon. It’s a judgement call but, ignoring the back story of how she came to be the hangwoman at Roscommon prison between 1780 and 1810, there might be enough on the tale to make it plausible.
Further to the Rainey Bethea case mentioned in the OP, Perry T. Ryan, a Kentucky DA, has written a fascinating book about it, The Last Public Hanging in America, the whole text of which he has put online. It’s well worth reading.