Hello, I am a US citizen in China on an L visa. I found a job in China.

Hi, you can try to convert your current L type to a work Z type at the exit and entry administration of the local PSB within China. If it can be done, you don't need to leave China.
You should first prepare the relevant documents including your passport and its copy, the application form, a passport-style photo, valid registration form of temporary residence, work permit as well as the other required documents. Then, submit the application to the local PSB.