[3][4][5] Thelen was known for its preeminent construction practice which dates back to the 1930s and the local association between the law firm and contractors and developers of large hydroelectric dam projects.

[6] In June 1998, Thelen Marrin merged with New York–based Reid & Priest, a firm with 160 attorneys founded in 1935 with a Washington, D.C., office, known for its industry experience in the electric utility sector.

Instead, it opted for domestic growth by acquiring the 10 lawyers of San Francisco–based boutique technology firm Britton Silberman & Cervantez in 2000.

[23] In late September 2008, twenty attorneys in the firm's China practice group in its Washington, D.C., and Shanghai offices left for Pillsbury Winthrop Shaw Pittman.

[29] In early November, between 60 and 90 attorneys from offices in San Francisco, Silicon Valley, New York, Los Angeles, Washington, D.C., and Shanghai were reportedly hired by Nixon Peabody.

On September 17, 2009, Thelen filed for Chapter 7 bankruptcy in the Southern District of New York, after the firm's only secured creditor, Citibank, was also no longer willing to advance funds for the cost of collection and to wind down operations.

[34][35] This subsequently ended the 85-year-old operation that peaked in the year 2006 with offices in a number of cities in the United States, England and China.