Prior to her election in 1982 to the Texas House District 98 seat, Shea had served for ten years as legislative assistant to Representative Bob Davis. In the House, she was the first woman to have served on the influential Ways and Means Committee. Her work for the House Insurance Committee earned her national recognition and election as president of the National Council of Insurance Legislators. She is also a recipient of the Texas Chamber of Commerce Legislative Leadership Award for outstanding public service. After redistricting in 1991 Shea was defeated by a 58-42 percent margin by Rep. Will Hartnett in the 1992 Republican primary run-off for State House District 114 after their districts were drawn together. Harnett won the general election and remains a state representative to this day.
In February 1993, Shea was appointed to serve as Dallas County constable for Precinct 2, including Irving, Coppell, and North Dallas. (All Texas counties have four such precincts regardless of population.) She was elected constable in 1994 (remaining two-years of a four-year term), 1996, and 2000. During her time as constable, Shea’s office collected more than an $1 million annually in hot check restitutions.
In 1995, Governor George W. Bush appointed Shea to the Texas Worker's Compensation Insurance Facility, which was thereafter privatized. In 1997, Bush named her as president of the Texas Healthy Kids Corporation, a public/private partnership to promote better health among the young.
Shea attended the University of North Texas in Denton and is a member of the University of North Texas System Board of Regents, under appointment from Governor Perry in 2007. She also studied at Dallas Baptist University. Shea is a former director of the Irving Chamber of Commerce and a past president of the chamber’s women’s division. She has also served on the advisory boards of the Irving Infant Intervention Center and Irving CARES. She is a member of the First Baptist Church in Irving.
The office of secretary of state in Texas is an administrative position concerned with keeping state election records and other documents. The office website maintains election returns since 1992 for national, state, and regional elections in Texas, but not local offices. Among well-known Texas politicians who have previously served as secretary of state are John Ben Shepperd, Mark White, John Luke Hill, Bob Bullock, Crawford Martin, Joseph Wilson Baines (grandfather of Lyndon B. Johnson), Ron Kirk, George Strake, Jr., Jack Rains, Alberto Gonzales, and Tony Garza.
On January 7, 2002, Shea succeeded the Democrat Henry Cuellar of Laredo in Webb County in South Texas, after Cuellar, himself a former state representative and Shea colleague, suddenly resigned after only nine months as secretary of state. Under Texas law, the secretary of state cannot engage in fund raising for another office. Cuellar left the post and thereafter ran unsuccessfully for the 23rd District seat in the United States House of Representatives against the Republican former Representative Henry Bonilla of San Antonio. In 2004, Cuellar won the 28th District seat in Congress and still holds the position.
In making his selection of Shea to succeed Cuellar, Perry described Shea as a "public servant of impeccable character and extraordinary energy. I have known her since we served together in the Texas Legislature, and she has proven herself as an outstanding leader and a committed public servant." For the swearing-in ceremony, Perry invited his future intraparty rival, U.S. Senator Kay Bailey Hutchison, who lauded Shea’s accomplishments: "Gwyn Shea has blazed trails throughout her life, and the milestone of becoming secretary of state is simply one more in her long and distinguished career. I am confident that she will be excellent in all of the responsibilities of this position." As secretary of state, Shea launched an initiative to reach graduating high school seniors with a voter registration kit. She unveiled the program in San Antonio in the spring of 2002. She also sponsored in elementary schools the essay contest "What Voting Means to Americans".
Shea resigned as secretary of state after less than two years in the position to accept employment with Harrah’s Entertainment Company. She lobbied for the privatization of the Texas state lottery. Shea was succeeded by Perry’s third choice for the position, Geoff Connor. Ironically, Connor had been the acting secretary of state for the three months between the Cuellar and Shea tenures, and he continued as deputy secretary of state under Shea. Connor in turn was succeeded as secretary of state by businessman Roger Williams, Phil Wilson, and Hope Andrade of San Antonio.
Shea's public papers have been deposited with the City of Irving. On her death, Shea will be interred beside her husband, John Joseph Shea (June 26, 1932–February 22, 1997), originally from Michigan, at the Texas State Cemetery in Austin. She has two children and two grandchildren.