On May 24, 2016, Miller was defeated in the Republican primary runoff election by Kyle Biedermann, who polled 10,481 votes (55.4 percent) to Miller's 8,438 (44.6 percent).
Miller in 1976 obtained a degree in law enforcement from Texas State University in San Marcos, north of New Braunfels on Interstate 35. He operates Miller & Miller Insurance. He and his wife, the former Anne Mund (born c. 1957), were once named "Small Business Persons of the Year" by the City of New Braunfels, of which he is a former mayor and city council member. The couple has two children, Douglas, II, the vice president of Miller & Miller, and Amanda, a political campaign consultant.
Miller sits on the board of directors of First State Bank in New Braunfels, the county seat of Comal County. He has also been heavily involved in matters of water development and conservation with service on the Region L State Water Planning Group and as a past chairman of the Edwards Aquifer Authority. Miller is a former president of the Comal County Fair Association, the Wurstfest festival (the New Braunfels equivalent of Oktoberfest), the Comal County United Way, and the Greater New Braunfels Chamber of Commerce. He is also a former president of the New Braunfels Junior Chamber International.
In the 2008 Republican primary election, Miller unseated the conservative one-term Representative Nathan Macias, a businessman from Bulverde, by 17 votes, 14,684 (50.02 percent) to 14,667 (49.97 percent). In the 2006 Republican primary Macias had unseated the former Republican representative, Carter Casteel, by forty-six votes. In the 2008 general election, Miller defeated a Democrat, Daniel Boone, 58,118 (69.4 percent) to 21,732 (26 percent). Another 4.6 percent of the vote went to the Libertarian Party nominee, Shannon Beckett McCracken.
In 2012, Miller won Republican re-nomination, 16,994 (75.1 percent) to Rob Smith's 5,633 votes (24.9 percent). Reelected in the 2012 general election with 88.2 percent of the group against another Libertarian candidate, Miller polled more raw votes, 64,029, than any other state House candidate in Texas that year.
In his third legislative session in 2013, Miller voted to forbid abortion after twenty weeks of gestation and to increase medical requirements and licensing of abortion providers. Texas Right to Life rated him overall 76 percent favorable in 2013, compared to 60 percent in his second term in 2011.
In 2013, Miller supported legislation to provide marshals for school security. He voted against the bill to authorize immunization of minors without parental consent, a measure which the House nevertheless approved, 71-61. He voted for the extension of the franchise tax exemption to certain businesses. He voted for the law which prohibits texting while driving. He voted to require testing for narcotics of those receiving unemployment compensation. He voted against an "equal pay for women" measure, which nevertheless passed the House, 78-61. He voted to forbid the state from enforcing federal regulations of firearms, and he co-sponsored the law which allows college and university officials to carry concealed weapons in the name of campus security. He voted against term limits for certain officials. He voted for the redistricting bills for the Texas House, the Texas State Senate, and the United States House of Representatives.
In 2013, Phyllis Schlafly's Eagle Forum, managed in Texas by Cathie Adams, a former state chairman of the Texas Republican Party who later broke with Schlafly over the Donald Trump candidacy, rated Miller 71 percent favorable, 25 points below the rating for preceding Representative Nathan Macias. Young Conservatives of Texas ranked him 64 percent. The interest group, Texans for Fiscal Responsibility, rated him 47 percent in 2013 and 50 percent in 2011. The Texas Association of Business gave Miller a cumulative score of 89 percent. He ranked 57 percent in 2013 from the Texas League of Conservation Voters; the Sierra Club rated him 38 percent favorable. The National Rifle Association rated him 92 percent.
An ally of Joe Straus, the Speaker of the Texas House of Representatives from San Antonio, Miller led two intra-party opponents in the Republican primary held on March 1, 2016. He received 18,509 votes (43.5 percent) to 16,963 (39.8 percent) for the runner-up, Kyle Biedermann. The remaining but potentially critical 7,100 votes (16.7 percent) went to the third contender, Chris Byrd.
Miller was defeated in the runoff by Biedermann.