Dentsu Aegis, which owns Carat, Vizeum and Mcgarrybowen, is taking part in the initiative to encourage more young people to vote.
Staff will be allowed to arrive later for work or leave early to make time to pop to their local polling station.
Publicis Groupe agencies, which include Publicis and Saatchi & Saatchi, are also allowing people to take time out of their working hours to vote.
Tracy De Groose, chief executive at Dentsu Aegis Network UK and Ireland, said there is a lack of turnout among younger voters.
She said: "It has been widely reported that turnout, especially among younger voters, could determine the outcome of the EU referendum.
"However, recent statistics by Ipsos Mori suggested that whilst there is 3.2 million more people aged 18-34 in the UK than over 65s, almost half are as likely to vote as those aged over 65.
"In an industry with a high proportion of employees under the age of 35, we are in a great position to encourage young people to have their say.
"This is one of the most important votes we will face in our lifetime and we must give our people the opportunity to have their say over our country’s future."
Adland haunt Soho House Group, which owns members clubs around London, as well as restaurant chains Pizza Pilgrims and Dirty Burger, is giving staff an hour at the beginning or end of their day to vote.
In an email to members, it also encouraged them to vote and spread the word to their family, friends and colleagues.