She is a dynamic diva-in-training who refuses to sing back-up, and is a member of the glee club at the fictional William McKinley High School in Lima, Ohio.

[11] When New Directions plans a benefit concert, Mercedes allows her performance slot to be moved to a less desirable place in the program; club member Lauren Zizes (Ashley Fink) is surprised by her failure to stand up for herself, and offers to be her agent, suggesting that she make diva-like demands to ensure they appreciate her talent.

[20] Sam continues to support her, and a YouTube video he posts of Mercedes singing results in her being offered a job as a backup singer in Los Angeles after she graduates.

She has moved to Los Angeles and is taking classes at UCLA while she works as a backup singer, though she returns to help with the school musical,[24] for Thanksgiving,[25] and for Mr. Schuester's ultimately failed wedding.

In the special 100th episode, due to their recent success, she and Rachel (who was just cast in a broadway role) starts a rivalry, though they made up with each other in the end as they realize they are equally talented.

She went on a mall tour across the state with her high school friends, Santana Lopez (Naya Rivera) and Brittany Pierce (Heather Morris) as her main back up.

She attends the McKinley auditorium rededication to Finn Hudson, sings one last song with every former member of New Directions, and takes a final bow with the rest of the Glee cast.

As Mercedes, Riley features in many ensemble musical performances, has had several solos and duets, which have been released as singles, available for download, and included on Glee's soundtrack albums.

Her first performance, "Respect" by Aretha Franklin, was included in full on the series' first DVD box set, Glee – Volume 1: Road to Sectionals.

[37] The rendition of Aguilera's "Beautiful" in the episode "Home" was called "so forced that it loses any power it might have from what's a genuinely nice arrangement of the song" by Emily VanDerWerff of The A.V.

[34] In the second-season episode "Grilled Cheesus", Mercedes performs Whitney Houston's "I Look to You", and led a choral rendition of Aretha Franklin's cover of "Bridge Over Troubled Water".

[44] Erica Futterman of Rolling Stone commended Riley's vocals on the latter, and commented that series creator Ryan Murphy should assign Mercedes more solos.

[45] MTV's Aly Semigran felt that "I Look to You" was the episode's weakest song, preferring Riley's "more powerful" rendition of "Bridge over Troubled Water".

[48] Mercedes later assumed the role of Frank N. Furter in the glee club's performance of The Rocky Horror Show, and sang "Sweet Transvestite".

[51] Anthony Benigno of the New York Daily News also enjoyed the number, and found that while Mercedes lacked the stage presence of Tim Curry, Riley did something "completely remarkable" as Frank, giving a largely flawless performance.

Co-written by Glee's music supervisor Adam Anders, composer Peer Åström, and executive producer Ian Brennan,[53] the piece peaked at number 53 on the US Billboard Hot 100.

[56][57] During the season, Mercedes reprised duets with both Santana and Rachel: with the former on Ike & Tina Turner's "River Deep – Mountain High"[58] and ABBA's "Dancing Queen",[59] and with the latter on "Take Me or Leave Me" from the musical Rent.

[34][61] "Take Me or Leave Me" featured on the following album, Glee: The Music, Volume 5, and reached number 51 in the U.S.[62][63] "Dancing Queen" appeared on the season's final soundtrack album, Volume 6,[57] and peaked at number 74 in the U.S.[64] Futterman criticized the arrangement of "Dancing Queen" for diluting Riley and Rivera's "power vocals", which, she said, "[gave] them the thin sound of ABBA's original even though they're both much better than that.

[66] Vanity Fair's Brett Berk praised her "amazing voice" and gave the performance four stars out of five, and The Hollywood Reporter's Lesley Goldberg characterized the rendition as "flawless".

"[70][71] In her "Maria-off" with Rachel, Benigno said they "both kill it", characterized it as "easy but fun", and gave it an "A−", as did Slezak and West, the latter of whom "didn't feel that Mercedes was the clear winner" but thought both singers were "fantastic".

[74][75][76] "It's All Over" from Dreamgirls received the most enthusiastic commentary, including an "A+" from West, who wrote "Mercedes was at her best in this performance", a sentiment echoed by Benigno when he awarded the song an "A".

[73][75] The "Mash Off" episode marked the 300th musical number Glee has filmed, which was a mash-up of two Adele songs: "Rumour Has It" and "Someone Like You", with lead vocals by Riley and Rivera.

Futterman wrote that the performance was one of Glee's best sequences: "Mercedes and Santana nail their vocals and the song combines great tracks from one of the year's biggest albums while capturing both the sass and sadness of the Troubletones at this particular moment.

[79] Jen Chaney of The Washington Post awarded the sequence a "A+" grade, and Brian Moyler of Gawker asserted that the number was "perfection" and said, "This is why I watch Glee.

Reviews were glowing: Reiter called it a "gorgeous, lush take" that was "definitely an episode highlight", and Bell described is as "a beautiful cover", "the perfect song for Mercedes to sing to Sam" and a "haunting performance".

[88][89] Chaney said that "Riley deserves a standing ovation", and that she sang the song with "a power motivated by genuine emotions that tied in directly to her storyline"; the "heartbreaking context of Houston's death ... immediately elevated this moment to the very best one of Glee's entire third season.

"[93] Stack approved of the budding friendship between Mercedes and Quinn in the episode "Home", and added, "Their scene together in the nurses office was really touching and sweet.

[94] Hyman hoped their coupling would continue,[95] though Matt Zoller Seitz of Salon.com lamented Sam's departure, as he felt a relationship between the two "would have been groundbreaking for all sorts of reasons".

"[71] Canning agreed, and wrote: "Schue's tough love with Mercedes came out of left field", and felt it added to the "inconsistency of these characters", even though "it ultimately made sense" in the context of setup for "It's All Over".