[4][5] Willis was selected by D.C. United with the 50th pick in the third round of the 2011 MLS SuperDraft, and he signed a contract with the team on March 16, 2011.

He made his MLS debut on August 13, 2011, recording a clean sheet in a 4–0 victory over Vancouver Whitecaps FC.

[10] Willis remained the second choice goalie for 2013, only getting an extended run of games while Bill Hamid was with the United States national team for the 2013 CONCACAF Gold Cup.

[14] His first appearance of the season with D.C. United came in a CONCACAF Champions League match against Waterhouse on August 20, a 1–0 win for D.C.

[16][17] On December 8, 2014, Willis was traded to the Houston Dynamo along with Samuel Inkoom, in exchange for the rights for Andrew Driver (who moved to Europe and didn’t play for D.C.) and a fourth-round pick in the 2016 MLS SuperDraft.

[34] He enjoyed a strong performance on July 21, saving a penalty kick and being named to the MLS Team of the Week after helping the Dynamo get a 1–1 draw against rival FC Dallas.

Willis started the Open Cup semifinal match up with LAFC on August 8 as the Dynamo won on penalties.

Willis kept a clean sheet in the final to help Houston defeat the Philadelphia Union 3–0 as the Dynamo won their first Open Cup in club history.

[39] On November 19, 2019, Willis was traded to Major League Soccer expansion side Nashville SC in exchange for Zarek Valentin.

[40] He made his debut for Nashville on February 29 in Nasvhville's first ever MLS game, a 2–1 loss to Atlanta United FC.

In a shortened season due to the COVID-19 Pandemic, Willis made 23 appearances, out of a possible 23, and kept a league best 9 clean sheets.

[42][43] Willis led a Nashville defense that conceded the 3rd fewest goals in MLS, leading the team to a 7th place finish in the Eastern Conference and qualifying for the playoffs.

[44] In Nashville's first ever playoff game, Willis kept a clean sheet to help defeat Inter Miami CF 3–0.