From 2009 to 2010, she appeared in various side characters in the Sketch show The Armstrong and Miller Show. In 2011, she appeared in the television improvisation show Fast and Loose.
Since 2011, Ransom has appeared as Morwenna Newcross, Doc Martin's medical receptionist in the ITV drama Doc Martin. Her character is known for her mismatched colorful outfits.
Ransom has run the London Marathon four times, in 2012 for Mencap, in 2014 for Age Uk and in 2015 and 2017 for Bloodwise.
In 2013, Ransom was in the Sky TV advert with Bruce Willis where she was "totally unlimited"; the advertisement was banned for being misleading, but has been re-released with more prominent information on the pricing.
From 2015 to 2016, Ransom appeared in series 4, 6 and 7 of Horrible Histories. In 2015, she won the Children’s BAFTA Award for her role as Mary, Queen of Scots in Horrible Histories.
In 2017, Ransom wrote an episode of Danger Mouse called "Live and Let Cry."
