Sullivan, who left the committee on Jan. 4, will join McGuireWoods on July 1, the firm announced Tuesday.
He was the right-hand man to Senate Finance Committee Chairman Max BaucusMax BaucusFive reasons why Tillerson is likely to get through Business groups express support for Branstad nomination The mysterious sealed opioid report fuels speculation MORE (D-Mont.) for more than a decade and served as staff director since 2004.
McGuireWoods Chairman Richard Cullen highlighted his role in tax and health policy.
"With his experience and keen insight into complex issues like taxation and changes in the healthcare market, it is no wonder that so many considered him the most sought-after staff member to leave Capitol Hill this past year," Cullen said.
Baucus announced earlier this year that he will not seek reelection in 2014. Before leaving, the chairman is focused on an ambitious overhaul of the tax code that has become a bonanza for K Street lobbying firms.