That’s 91 days longer than the hospital in Phoenix reported, says the inspector general for the Department of Veterans Affairs (VA).
It also said at least 1,700 veterans were not even on waiting lists because they were not properly registered.
President Barack Obama called the findings “deeply troubling”.
“We have substantiated that significant delays in access to care negatively impacted the quality of care at this medical facility,” acting inspector general Richard Griffin wrote in the report.
Investigators reviewing a sample of 226 patients found that while their average wait time for a first appointment was 115 days, the medical centre instead reported it as 24 days.
Arizona Senator John McCain has called on Veterans Secretary Eric Shinseki to resign.
Mr Shinseki called the report’s findings “reprehensible to me, to this department and to veterans”, and said he would direct the hospital to immediately address each of the veterans waiting for medical appointments.
The White House has come under growing pressure from veterans groups, congressional Republicans and the media after it was alleged that officials at the Phoenix VA hospital maintained secret records to avoid reporting to Washington the true length of time veterans had to wait for appointments.
Last month, a retired doctor at that hospital told broadcaster CNN he believed as many as 40 veterans had died while waiting months to be seen.
Last week, President Barack Obama acknowledged veterans had to wait too long to receive medical care and vowed to punish any dishonourable misconduct related to the scandal.
“When I hear allegations of misconduct, any misconduct, whether it’s allegations of VA staff covering up long wait times or cooking the books, I will not stand for it, not only as commander in chief but also as an American,” Mr Obama said in a White House press conference on 21 May.
One day prior to Mr Obama’s comments, the VA inspector general’s office said 26 veterans hospitals and facilities were being investigated.
But investigators have said they have not yet linked any patient deaths to the delays in care.
The recent allegations also come amid ongoing issues at the VA, including a lengthy backlog in disability claims and high veteran unemployment.
Earlier this month, the American Legion, a prominent veterans group, and the Army Times, an influential independent newspaper, called on Mr Shinseki to step down.
At an appearance before a Senate panel earlier this month, Mr Shinseki said he was “mad as hell” over the allegations but was waiting for the inspector general’s report before taking action.
“Whatever comes out of this, whatever is substantiated, we will take action,” he told reporters after that hearing.