Robertson joins the leading west London events agency as a graduate recruit to the O2 account working on its conferences and experiential events.
She graduated from Cardiff University, studying media and journalism, and prior to her appointment by Sledge, Robertson completed an internship with the agency, working on Olympics projects.
Sarah Walker has also joined Sledge as senior account manager on the O2 account.
She was previously at TRO working on the Honda and Chevrolet accounts.
Sledge managing director Laura Capell said: "It's a really exciting time for Sledge and we're thrilled to have two new recruits in addition to the existing team.
"Sarah, an experienced and seasoned events professional, and Holly who after an initial stint of work experience over the summer, has proved herself a true Sledgeling and fitted perfectly into the agency - we are confident she has what it takes to go far, particularly at Sledge."
O2 is this week sponsoring the Wired 2012 event at The Brewery, where around 500 delegates including entrepreneurs and investors join to learn from influential business speakers.