Born in Texarkana, Arkansas, Sandlin earned Bachelor of Arts and Juris Doctor degrees from Baylor University, where he was selected Outstanding Young Alumni. His political career began when he was elected county judge (chief executive) of Harrison County in 1986. In only one term, he balanced the county budget and left office with a budget surplus. He also cut ad valorem taxes. In 1989 was subsequently elected Judge of the County Court at Law in the county seat of Marshall, serving there until his election to Congress in 1996.

Sandlin served on the House Transportation and Infrastructure, House Financial Services, Ways and Means committees. In 2002, Sandlin became a chief deputy whip for the House Democrats.

Sandlin was a leading critic of the 2003 redistricting orchestrated by then-House Majority Leader Tom DeLay. His district was largely rural and had been throughout its history, but the 2003 redistricting cut out Texarkana and several rural areas that had been part of the district for more than a century. In their place, it added the heavily Republican cities of Tyler and Longview. Sandlin claimed that such a remap silenced unfairly the voices of rural and minority Texans. After considering a run against Democrat-turned-Republican Ralph Hall in neighboring District 4, which had absorbed several counties of his former district, he decided to run again in District 1 in January 2004. He was defeated by Republican Louie Gohmert in the November election.

Sandlin continues to advocate that the 2003 redistricting process was illegal and should be ruled as unconstitutional. An unconstitutional ruling would cause a special election to take place in either the old district drawn by a court in 2000 and used in the 2002 elections, or in a newly drawn district. Sandlin maintains that he is keeping all of his options open in 2006 and beyond.