The 68-year-old McMahon was a major Trump backer during his campaign, giving $6 million to a pro-Trump super PAC.
The SBA has an office in every state and helps with financing and training for small businesses and entrepreneurs. President-elect Trump said in a statement that McMahon would aim to "bring back our jobs and roll back the burdensome regulations that are hurting our middle class."
"Our small businesses are the largest source of job creation in our country," McMahon said in a statement. "I am honored to join the incredibly impressive economic team that President-elect Trump has assembled to ensure that we promote our country's small businesses and help them grow and thrive."
McMahon tweet: Honored to be appointed by President-Elect @realDonaldTrump to serve as head of @SBAgov advocating for our small businesses & entrepreneurs!
McMahon ran unsuccessfully for U.S. Senate seats in Connecticut twice in 2010 and 2012. She spent nearly $100 million on the two campaigns.
In her campaigns, McMahon supported a lower corporate tax rate and a reduction in regulations. Trump has proposed chopping the corporate tax rate to 15 percent from 35 percent and drastically rolling back regulations.
McMahon and husband Vince founded WWE more than 30 years ago. The publicly traded company now has a market value of about $1.5 billion.
Trump has ties to the WWE and is a member of its hall of fame.