Following her retirement from teaching in 1976, Lee became involved in Fort Worth community causes. Lee helped found the Tarrant County Black Historical and Genealogical Society, alongside civil rights activist Lenora Rolla. She helped organize Fort Worth's annual Juneteenth celebration. Lee helped campaign for Ann Richards, Texas Gubernatorial candidate and Presidential candidate Barack Obama. During the 1980s, Lee originated a tradition of bringing Fort Worth city leaders on an annual bus tour through economically depressed areas of Fort Worth, pointing out landmarks important to Fort Worth's minority communities. Her nonprofit organization, Unity Unlimited Inc., has been in operation since 1994 and was officially incorporated in 2000.

Lee talks with President Joe Biden after the signing of the Juneteenth National Independence Day Act Bill.

Lee campaigned for decades to make Juneteenth a federal holiday. She promoted the idea by leading 2.5 miles (4.0 km) walks each year, representing the 2.5 years it took for news of the Emancipation Proclamation to reach Texas. At the age of 89, she conducted a symbolic walk from Fort Worth, from which she departed in September 2016, to Washington, D.C., where she arrived in January 2017. She was hoping to plead the case for a federal holiday directly to President Barack Obama. Lee has not only marched in Texas, but also in Fort Smith and Little Rock, Arkansas; Las Vegas, Nevada; Madison and Milwaukee, Wisconsin; Atlanta, Georgia; Selma, Alabama; and the Carolinas She promoted a petition for a Juneteenth federal holiday at Change.org; the petition received 1.6 million signatures. She said, "It's going to be a national holiday, I have no doubt about it. My point is let's make it a holiday in my lifetime."

In June 2021, at the age of 94, her efforts succeeded as a bill to make Juneteenth a federal holiday was passed by Congress and signed into law by President Joe Biden. She was an honored guest at the bill signing ceremony, receiving the first of many pens Biden used to sign the document. As she sat in the front row, she received a standing ovation and Biden got down on one knee to greet her.

Lee is also a founding board member of Transform 1012 N. Main Street, a coalition of Fort Worth nonprofit and arts organizations working to turn a former Ku Klux Klanauditorium into the Fred Rouse Center and Museum for Arts and Community Healing. Rouse, a Black man, was lynched by a Fort Worth mob in 1921. Transform 1012 N. Main Street was formed in 2019, and announced the acquisition of the building in January 2022.

Lee was named the 2021 "Texan of the Year" by The Dallas Morning News for her activism on behalf of Black Texans. She was also included in the 2021 book Unsung Heroes for operating a food bank, farm, and community garden throughout the COVID-19 pandemic. In 2024, Lee was awarded the Presidential Medal of Freedom by President Joe Biden.