The agency won the 12-month account following a competitive pitch. It is the first time the charity has retained an agency.
Geronimo will promote Marriage Care's services throughout the UK and work to attract new volunteers to become counsellors.
The charity was established in 1946 as the Catholic Marriage Advisory Council. It changed its name to Marriage Care in 1995.
Marriage Care wants to break down misconceptions that its services are exclusive to Catholics, stressing that gays, lesbians and people of all faiths are welcome.
Although the organisation still believes that 'marriage is a vocation of love', it is not prescriptive. Roughly a quarter of Marriage Care's clients are Catholic.
'Marriage Care uses a client-centred approach and prospective counsellors go through an intensive selection programme to ensure they are not people who will push their views on those being counselled,' said director of training Joe Mannion.
Marriage Care is hoping to attract a younger audience to use its services as well as to counsel couples. It will also promote its marriage preparation courses.
Geronimo will focus on targeting the national press and women's consumer titles. It will stress that the service is useful for couples at any stage of their relationship, not just when it breaks down. The Geronimo team will be led by director Anastasia Scott, who will report closely to CEO Terry Prendergast.
Marriage Care was originally set up as a response to the breakdown of marriages in the aftermath of World War Two.
Prendergast said: 'Sixty years on, couples face different social and financial pressures, but the importance of having a relationship support service remains vital. The main objective for us is to ensure as many people as possible have full knowledge and understanding of the services we provide.'
The organisation receives about a third of its income from the Department for Children, Schools and Families. The Catholic Church provides some accommodation for the group, but does not provide funding.