Marketeer CEO Keith Searby said a deal had been planned for around six months and would complete by 1 May.
He said the impetus came from both having a desire to create 'critical mass' to handle larger clients and to save money on overheads such as rent and HR.
Both Searby and McCluskey MD David Rose said there would be no redundancies as a result of the deal, which will see co-location in McCluskey's offices in Hammersmith, west London.
Both agencies will retain their brand identities, although an umbrella group will be created with four directors: from McCluskey, principal Judy McCluskey and Rose, and from Marketeer, Searby and finance director Sue Newton.
The combined entity will have annual fee income of more than £2m and more than 30 staff, drawn almost evenly from each agency.