In it, he asks the prime minister to "honestly acknowledge the crisis," and "accept responsibility for the fact the status quo isn’t working."
The letter was released following news of two suicides of Canadian Armed Forces members over the holiday break.
Mulcair's letter, dated Jan. 10, says that while he appreciates steps made to improve access to health services, "it is clear that these efforts have not been sufficient."
The NDP leader mentions the more than 50 boards of inquiry on military suicides that remain outstanding, as well as a still unreleased report from an inquiry into the 2008 suicide of Cpl. Stuart Langridge.
"Canadians are left with grave concerns about whether the system put in place to help our Armed Forces is broken, or if we are learning from these tragedies in order to prevent future ones," he wrote.
Before the rash of deaths leading up to the year's end, the military had calculated six suicides in 2013. Adding the five recent deaths brings the total to 11, a number on par with recent years.
Liberal MP Jim Karygiannis also released a statement earlier this week expressing condolences for the victims' families.
"The issue has been constantly raised with the departments of National Defence and Veterans Affairs," Karygiannis wrote.
"Little has been done to date to address the need to look after our veterans and military personnel."
In 2012, the government allocated an additional $11.4 million to the Department of National Defence's mental health budget, bringing the total to $50 million.