Lehman was born in Fresno, California and attended local public schools. He attended Fresno City College before transferring to the University of California at Santa Cruz from which he earned his degree. Upon graduating from college, he enlisted as a reservist in the National Guard and received a job as a staff member for California State Senator George Zenovich. In 1976, Lehman was elected to the State Assembly as a Democrat. He rose quickly through the ranks, becoming assistant majority leader by 1978. In 1983, he successfully ran for a seat in Congress, representing a newly-created district in Fresno. He was reelected five times.
Lehman was known in Congress for his passionate commitments to important causes such as environmental protection, recognition of the Armenian Genocide, and gun control. He was able to strike delicate balances on many of these issues, a particularly important skill given the increasingly conservative nature of his Fresno-based congressional district. His notable successes include playing instrumental roles in passage of laws to protect the Kings and Tuolumne Rivers, the California Desert Protection Act, and expansions of wilderness areas in the Sequoia and Yosemite National Parks.
Lehman's district was made considerably more Republican after the 1990s' round of redistricting. It was renumbered the 19th District, and pushed further to the north, incorporating some more rural territory in the Sierra Nevada. At the same time, he lost his share of Stockton to the new 10th District, and lost most of his Latino constituents to the neighboring 20th District. Leahman barely held onto his seat in 1992, winning by only 1,100 votes over Republican Tal Cloud, a businessman from Fresno. This led to speculation that the 19th would not stay Democratic for long. In 1994, Lehman was swept from office in the Republican Revolution, losing to Republican George Radanovich by a shocking 17-point margin--one of the largest margins of defeat for an incumbent that year.
Lehman is currently a partner for the Sacramento-based lobbying firm Lehman and English. He also serves as a member of the board of the National Parks Trusts, the Mono Lakes Committee, and is on the board of directors of Premier Valley Bank in Fresno.