The Wall Street giant, which has been buffeted by analyst scandals over the last year, is expected to take up to six months finding a replacement for the outgoing SV-P for communications and public affairs, who has been with the group for 18 years.
Critchlow's announcement follows a troubled time for the group.
Merrill was the first Wall Street firm to come to the attention of US regulators for alleged deception by its sell-side research department and has been criticised for its alleged role in enabling Enron to hide its true financial condition from investors.
The group has also seen a recent executive reshuffling under new CEO Stanley O'Neal, but Critchlow said speculation that his departure was linked to board changes was unfounded.
He said: 'We have a very close relationship. It's just the right time for a change, both for me personally and for this position.'
Critchlow had survived two previous CEO transitions prior to O'Neal's accession, and said he has been contemplating retirement for over a year.
He added that the firm's recent difficulties played no role in his decision.
'Sure it's been a difficult year in some ways, but I've been through tough times before,' said Critchlow. 'After 18 years, I already had lots of scar tissue.'
Critchlow is set to retain his title throughout a transition period that is expected to take 12 to 18 months. He joined Merrill in 1985, and became head of PR in 1988.