The authorization was set to expire at the end of the year as states have relied on National Guard members to run testing sites, deliver supplies, stock food banks and sanitize facilities.
"To date, activated National Guard forces around the country have provided critical support to governors," the memo stated.
"The states and territories will need assistance in fighting hot spots as they emerge."
The rate of federal funding for the National Guard will remain at 75%, leaving each state to pay the remaining cost.
Additionally, as of Dec. 31 the federal government will end a special provision to pay 100% of the cost share for National Guard forces in Florida and Texas and both states will receive only 75% moving forward.
The extension applies to all 50 states as well as the territories of Guam, Puerto Rico and the U.S. Virgin Islands.