Calver joins the firm from TNS in Edinburgh, where she held the post of managing consultant and worked for clients including STV and Guardian Media Group.
At Essential she will work within the firm's qualitative research team. Before her time at TNS, Calver worked as a researcher at Cragg Ross Dawson in London.
Alex Charlton, Essential's director, said: "The scale and expertise of our team is growing fast and we're very proud to have attracted a colleague of Sarah's calibre to our agency.
"It's important for us to make sure we continue to hire the best staff to deliver the best quality research in the media industry and Sarah will be a great addition to our team in this respect."
Calver added: "I'm really excited about the approach Essential takes to research and the way it works with clients and I'm looking forward to getting involved in some really interesting projects as soon as possible."