The formal policy document comes a month after Trump surprised the Pentagon when he announced in a series of July 26 tweets that he would reverse the Obama-era policy change.
"The President's willingness to attack his own troops is unprecedented and poses a serious threat to our nation's longstanding commitment to honor those who serve and to prioritize military readiness over politics", said Minter in a statement.
President Donald Trump plans to give the Pentagon six months to develop a policy to ban transgender personnel from the military, despite pushback from current commanders and strong criticism from former generals and admirals.
According to Trump's memo, transgender people can not join the military from now to beyond January 1, the deadline for when the military was to begin admitting transgender recruits. "It appears that Mattis has discretion substantively and procedurally". "This policy is based on a series of national security considerations".
This memo also puts a stop on funds which are used for sex-reassignment surgery for the people who are active.
The memo called on Mattis to submit his plan on how to implement the changes by February 21 of next year.
According to CNN, the White House offered no instructions as to how the ban would be implemented.
A White House official who briefed reporters about the memo declined to specify whether transgender individuals who are now active in the military could continue to serve based on such criteria.
But Trump suggested his predecessor failed to think through the issue of whether allowing transgender individuals to serve opently would "hinder military effectiveness and lethality, disrupt unit cohesion, or tax military resources".
Mattis said late in June that the Pentagon needs more time to assess whether the new policy would affect the ability of the USA military to defend the country.
The Hill also reported that "approximately 250 transgender people now" serve "openly in the US military".
California Democrat Ro Khanna tweeted: "Our transgender service members deserve honour and respect". While Castro wouldn't predict whether the ban will end up going into effect, he did note that - for the most part - transgender people are mostly excluded from the military as it is.
A recent Reuters poll found that by a more than 2-1 margin, the American people support allowing transgender persons to serve in the military.
Trump's memo frames Obama's policy - which was crafted after a working group spent a year to hammer out the details and implementation - as dismantling the Pentagon's "established framework".