Trahan was born in Abbeville in Vermilion Parish. He graduated with two bachelor's degrees in 1971 and 1977 from the University of Louisiana at Lafayette. He has been vice president of the Investment Company Institute, an insurance company. He is a former director of the Lafayette Zoning and Development Commission. He was an assistant to the former Louisiana Public Service Commissioner Jimmy Field, a Republican from Baton Rouge. He was finance chairman of the Louisiana Top 28 School Basketball tournament. He is Roman Catholic and has four children, Karen, Sarah, Mary, and John Trahan.
In 2003, Trahan finished second in the nonpartisan blanket primary for the District 31 House seat. He finished with 4,063 votes (27.3 percent), compared to the top vote-getter of the four candidates, Republican Charles Lee "Charlie" Buckels, Jr. (born December 1945), of Lafayette, who polled 4,380 votes (29.5 percent). Two other candidates, Republican Chad Zerangue and Democrat Neil J. "Sam" Melancon, both finished with just under 22 percent of the votes cast. In the runoff contest on November 15, when Kathleen Blanco defeated Bobby Jindal for governor, Trahan defeated Buckels by 14 votes, 8,181 (50.04 percent) to 8,168 (49.96 percent).
In 2007, Trahan won his second term by 20 votes more than he had in 2003, a margin of 34 votes, 7,365 (50.1 percent) to Nancy Landry's 7,332 (49.9 percent).At the time Landry was an Independent. When Trahan resigned, Landry switched to Republican affiliation to contest the special election for the remainder of his term. She polled 13,463 votes (62.2 percent) to Republican Troy Theriot's 8,197 (37.8 percent).
Trahan's legislative ratings ranged from 86 to 94 percent favorable from the Louisiana Association of Business and Industry, 89 percent from the National Federation of Independent Business, 100 percent from Louisiana Right to Life, 71 to 100 percent from the conservative Louisiana Family Forum, 64 percent from the Humane Society, and 50 percent from the Louisiana Hospital Association.
During his last year in office, Trahan voted against the castration of sex offenders. He opposed a legislative pay increase but supported higher pay for statewide officials. He voted to restrict gifts to elected officials. He voted to ban public funds for human cloning. He supported deregulation of the cable television industry. He voted to increase both college tuition and automobile insurance rates. He voted against the anti-bullying bill which supporters claimed would have made the practice less likely in public schools. He voted to allow supplementary science materials in public schools; only three House members opposed that measure.
Donald Mark "Don" Trahan