Miller worked as a management consultant at Accenture, as an event planner[3] and as an account vice president at UBS Wealth Management in Denver.[4]
Together with his husband, Tim Gill, Miller is active in LGBT rights activism, philanthropy, and Democratic Party politics.[4]
Both are co-chairs of the Gill Foundation,[5] one of the largest sponsors of LGBT equality causes in the United States.[4] The foundation was instrumental in improving the reputation and visibility of LGBT people in Colorado and changing its image as a "hate state".[5] During the COVID-19 pandemic, Miller led programs that provided more than 5 million meals to Coloradans in need.[6][7]
Gill and Miller are political allies of Colorado Governor Jared Polis, and Gill has been described as "one of the architects of the Democratic takeover of Colorado politics".[4] Gill and Miller have donated at least $3.6 million to Democratic candidates and campaigns since 2010,[3] and Miller has been active in groups supporting the presidential candidacies of Hillary Clinton and Joe Biden.[4]
In May 2025, TIME Magazine listed Miller and Gill as "Titans" on its TIME 100 Philanthropy 2025 list.[8]
On August 6, 2021, President Joe Biden announced his intent to nominate Miller to be the U.S. Ambassador to Switzerland and Liechtenstein. The post is traditionally given to a political appointee, often a prominent donor.[3][9] Notable LGBTQ+ organizations like Victory Fund praised Miller's nomination.[10]
On August 10, 2021, his nomination was sent to the Senate.[11] Hearings on his nomination were held before the Senate Foreign Relations Committee on November 2, 2021. In his prepared testimony, Miller highlighted priorities that would guide his work if confirmed.[12] The committee reported him favorably on December 15, 2021.
On December 18, 2021, the United States Senate confirmed his nomination by voice vote.[13] He was sworn in on December 21, 2021.[14] He presented his letters of credence to the President of the Swiss Confederation, Ignazio Cassis, on January 11, 2022.[15] On February 16, 2022, he presented his credentials to Alois, Hereditary Prince of Liechtenstein at Vaduz Castle.[16][17] Miller is one of a small number of openly gay U.S. Ambassadors.[18]
