"I spoke to the playing group and the entire staff this morning, and outlined my passion for football and the opportunities I see in the role," Lethlean said.
"I believe I can be part of a team that delivers real and sustained success to St Kilda Football Club."
Lethlean resigned from the AFL in July after news emerged he had been involved in an affair with a fellow employee.
"People make mistakes and people need to be accountable for their mistakes, but ultimately we're really confident that Simon has demonstrated through his attitude, through the experience that he's had, that he recognises the mistakes that he's made," Saints CEO Matt Finnis said.
"He's put his hand up and we're willing to give him a second chance."