The actress played The Doctor's companion Martha Jones in Doctor Who and Torchwood, but is now working in the US on The Carrie Diaries.
Speaking to Assignment X, Agyeman admitted that she has been amazed by how popular Doctor Who has become in the US.
"I am so overwhelmed and so honoured and pleased," Agyeman said of the show's popularity. "I walk down the street here and there are Doctor Who fans coming up to me, and I can't believe it."
She went on to say: "I can't believe how it has just exploded over here. When I was doing the show, we didn't get to come over to America and do any of the promotion.
"The new lot [of Doctor Who actors] did, but my agent said, 'Don't worry, whatever happens with them, it kind of ripples outwards and the whole family reaps the benefit of that', so it's been great for my career."
Agyeman recently ruled out playing Martha Jones again in Doctor Who's 50th anniversary, but said she might take part in promotional events.
The BBC also recently announced that 9th Doctor actor Christopher Eccleston will also not appear in the anniversary episode.
Doctor Who's anniversary celebration will include current Doctor Matt Smith, Jenna-Louise Coleman, past stars David Tennant and Billie Piper and classic villains the Zygons.
Watch the stars of Doctor Who talk the 50th anniversary special with Digital Spy below: