The duo have agreed two-year deals with the Whites and will complete their moves when their respective contracts with Rotherham United and Bolton Wanderers expire at the end of the month.
The club will also retain the option of an additional one-year extension that will keep them in SW6 until at least 2017.
Midfielder Pringle told the club’s website: “I’m buzzing. As soon as I knew that there was interest from Fulham I wanted to come here straight away.
“You just have to look at the size of the club and where they’ve been in the past – hopefully we can get there again before too long.”
Goalkeeper Lonergan as a former England U21 international and is understood to have been shown the door as part of a cost-cutting exercise by Bolton despite making 32 appearances last term.
He moved across the Pennines from Yorkshire in 2012 after leaving Leeds United, having previously been named player of the year twice at his hometown club Preston.
Lonergan added: “I’m delighted to get it all sorted. I’m happy to be here and I can’t wait to get started. It’s a very well respected, historic club and it’s a place where everyone would like to play.
“We have a lot of potential to hopefully get back to where the club belongs."