Yes, co-signing a mortgage will affect your credit. Even if the borrower stays current on their payments, co-signing can increase your DTI, making it more difficult to take out loans of your own. Though in some cases, lenders may look at co-signed debt differently. "There are circumstances where the mortgage can be omitted, but they would be required to show six to 12 months of satisfactory payments from someone else's account," Shayowitz says.  Any negative information associated with the loan, including late or missed payments, will show up on your credit report, as well.