Leucadia, which has been called a mini Berkshire Hathaway for its holdings ranging from real estate to mining, already holds about 29% of Jefferies.
Under the agreement, Jefferies shareholders will receive 0.81 Leucadia share for each share held, or $17.66 based on Friday closings, representing a premium of 24 percent to Jefferies Friday closing price of $14.27.
Jefferies shares were trading at $17.40 before the bell on Monday, while Leucadia shares were up 3.4 percent at $22.55.
Jefferies Chief Executive Richard Handler will lead the combined company while current Leucadia Chairman and Chief Executive Ian Cumming will retire.
The deal, expected to close during the first quarter of 2013, will see Jefferies shareholders owning about 35% of Leucadias common stock.