Details are in financial disclosure forms released today.
The forms show that Kirk made more than $1 million last year from his law firm partnership and corporate board positions.
He's been a partner at Houston-based law firm Vinson & Elkins since February 2005.
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Vinson & Elkins will pay him a bonus of $150,000 and other benefits, when he resigns.
President Barack Obama last month named Kirk to be the nation's next lead trade negotiator.
Kirk was elected the first black mayor of Dallas in 1995 and re-elected in 1999. He ran for the U.S. Senate in 2001, but lost to Republican John Cornyn, who still holds the Senate seat.