For his role as Astrov in the Lincoln Center Theatre revival of Uncle Vanya (2024) he earned a nomination for the Tony Award for Best Actor in a Play.

[9][10] In 2008 he performed in Manhattan Theatre Club's production of Lynn Nottage's play, Ruined, which won the 2009 Pulitzer Prize for Drama.

[15] He starred as Marty Boy in debut of The Total Bent, a musical composed by Stew and Heidi Rodewald, that premiered at The Public Theater in 2012.

[18] Harper made his Broadway debut in 2014 portraying Stokely Carmichael in the Robert Schenkkan play All the Way starring Bryan Cranston as President Lyndon B. Johnson.

[20][21][22] Marilyn Stasio of Variety praised him writing, "Harper inhabits the difficult role as comfortably as someone who lives in robe and slippers".

[29][30] In 2017, in between filming the show's first and second seasons, he had a leading role alongside Cristin Milioti in Zoe Kazan's futuristic romantic drama After the Blast at Lincoln Center's Claire Tow Theatre.

[31][32] Frank Scheck of The Hollywood Reporter wrote, "[Kazan] has...elicited superb performances from the lead actors, with Harper quietly persuasive as the husband desperately trying to keep his marriage intact".

[34] In an interview with Bloomberg News, he revealed that the play was inspired by the displacement of a poor Black community that was situated near the grounds of the Texas State Fair in Dallas.

[37][38] Also that year, he voiced John Mercer Langston in season one of the Airship podcast 1865, which details the weeks immediately following the assassination of US President Abraham Lincoln.

[41][42] In 2021 he acted in the Barry Jenkins directed Amazon Prime Video miniseries The Underground Railroad adapted from the Pulitzer Prize-winning 2016 historical novel of the same name by Colson Whitehead.