She rose to fame with her role as the Doctor's companion Martha Jones in the BBC science fiction series Doctor Who (2007–2010) and its spin-off Torchwood (2008), and received further recognition for playing Crown Prosecutor Alesha Phillips in the ITV crime procedural Law & Order: UK (2009–2012), Amanita Caplan in the Netflix science fiction drama Sense8 (2015–2018) and Dr. Helen Sharpe in the NBC medical drama New Amsterdam (2018–2023).

In 2013, she made her US television debut in The CW's Sex and the City prequel series, The Carrie Diaries, as magazine editor Larissa Loughlin.

Later, during the summer of 1996, she attended the Anna Scher Theatre School in Islington and studied performing arts and drama at Middlesex University, graduating in 2000.

[7] Before securing the part of Martha Jones in Doctor Who, Agyeman's most famous television role was playing Lola Wise in the revived series of the ITV soap opera Crossroads.

Agyeman made her feature film debut in 2006, playing Nana in the independent drama Rulers and Dealers (2006), written and directed by Stephen Lloyd Jackson.

On 24 June 2005, she auditioned for the part of Sally Jacobs in "The Christmas Invasion", but the role went to Anita Briem, who better fitted the production team's concept of the character as an "ice cool blonde".

Agyeman studied dance and practised horse riding, martial arts and gymnastics, a fact that the producers exploited to provide her with more physically arduous action scenes.

She attended another audition (ostensibly for an episode of Torchwood)[11] before a final screen test with Tenth Doctor actor David Tennant.

Agyeman also voiced the character in the animated serial The Infinite Quest, which aired in twelve weekly segments during the run of the 2007 series.

In between the third and fourth series of Doctor Who, Agyeman appeared in three episodes of the show's spin-off drama Torchwood,[27] namely "Reset", "Dead Man Walking" and "A Day in the Death".

[28] It was later revealed by director Euros Lyn that the production team had intended for Agyeman to also appear in the third series of Torchwood, but she was already committed to Law & Order: UK.

Agyeman played foundling girl Tattycoram in the Emmy Award-winning 2008 BBC adaptation of Charles Dickens's Little Dorrit, which began on 26 October 2008, starring alongside her fellow Torchwood co-stars Eve Myles and Ruth Jones.

In preparation for the role, Agyeman sat in on live trials, toured the Old Bailey, and conferred with her law-degree-holding sister, Leila: "She's great and really patient at explaining things to laymen, because I was coming at it completely blind.

[46] In March 2012, it was announced that Agyeman would play Larissa, "an easy-going party girl who works at Interview magazine" in the pilot of the Sex and the City prequel The Carrie Diaries (2013–2014).

[52][49] In the same year, she starred as Penny, a woman embroiled in a crime war between northern and southern UK gangs, in the film North v South.

[56] In 2018, Agyeman became a member of the main cast of the NBC medical drama series New Amsterdam, playing the role of Dr. Helen Sharpe, the head of the oncology and hematology department.

In July 2022, she announced that she would not be returning to New Amsterdam for its fifth and final season,[57] instead signing up for Sky Max's comedy-drama Dreamland, which also featured singer Lily Allen in her first television lead role.

[58] The tattoo Agyeman has on her upper arm is symbolic of her Iranian ancestry, containing the Persian/Iranian word "raha", meaning "free", under an image of a butterfly.