Anthony Senecal wrote in a Facebook post that Mr Obama "should have been taken out by our military and shot as an enemy agent in his first term".
Mr Senecal worked for Mr Trump, the presumptive Republican nominee for president, for nearly 30 years.
The Trump campaign quickly disavowed Mr Senecal's remarks.
"He is not employed by Mr Trump, and hasn't been since June of 2009," Trump campaign spokeswoman Hope Hicks said in a statement. "We strongly condemn these horrible comments from Mr Senecal."
Mr Senecal's post, which was not public, was first reported by Mother Jones, but he later confirmed its authenticity to several news organisations.
Later on Thursday, Mr Senecal, 84, told CNN that Mr Obama should be "hung" outside the White House. He also called the White House the "White Mosque".
The New York Times profiled Mr Senecal in March, saying despite retiring in 2009 he has stayed at Mr Trump's Mar-a-Lago estate in Florida as "a kind of unofficial historian".
"He understands Mr Trump's sleeping patterns and how he likes his steak ("It would rock on the plate, it was so well done"), and how Mr Trump insists - despite the hair salon on the premises - on doing his own hair," the profile read.
The US Secret Service has launched an investigation after Donald Trump's former butler called for the death of President Barack Obama.