Before working under Kelly while he was secretary, Nielsen worked in one of DHS's agencies, the Transportation Security Administration (TSA), and served former President George W. Bush's Homeland Security Council.
But Nielsen had one crucial advantage - the absolute trust and support of Kelly, who she grew close to after volunteering to help "sherpa" him through the confirmation process earlier this year.
President Donald Trump is expected to nominate Kirstjen Nielsen to be Secretary of Department of Homeland Security Thursday, two senior sources with direct knowledge of the nomination told ABC News.
Her detractors viewed her as brusque and, at times, unresponsive, as she worked with Kelly to streamline West Wing operations and instill discipline in a White House often void of it.More news: Trump Discusses 'Range of Options' to Respond to North Korean Aggression
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The White House says Nielsen has "extensive professional experience in the areas of homeland security policy and strategy, cybersecurity, critical infrastructure and emergency management".
Former DHS Secretary Tom Ridge issued a statement praising Nielsen on Wednesday, calling her a "homeland security veteran" and saying: "Kirstjen can hit the ground running and there won't be a learning curve". They spoke on the condition of anonymity since the decision has not been formally announced.
That's according to three people familiar with his decision. Elaine Duke has been filling in as acting secretary in the meantime.
At the White House, however, as Kelly's enforcer, Nielsen quickly emerged as a controversial presence.