He joined Yisrael Beiteinu and ran for mayor of Dimona in 2003, and was elected with 42% of the vote. He was elected for a second term in 2008, and was expected to win a third term in 2013. During his tenure numerous factories opened, the city's first mall was built, unemployment dropped, he built cultural and youth centers, and led the efforts to lower the price of water. In October 2012 he joined Yesh Atid after being courted by the party for several months.
In addition to serving as mayor of Dimona, Cohen was also a member of the board of directors of the Jewish Agency.
In the lead-up to the 2013 Knesset elections Cohen was placed fourth on the list of the new Yesh Atid party, entering the Knesset as the party won 19 seats. He was appointed Minister of Welfare and Social Services, a post he held until resigning on 2 December 2014. He was placed fourth on the party's list for the 2015 elections, and was re-elected as the party won 11 seats.