He has led the organisation, formerly the Association of London Government, since 1997. He said it now needed a chief executive who would serve a full four-year term with its new multi-party executive.
He said: 'It may sound a cliché but I'm looking for a portfolio career of some sort, and it's time for me to do some pro-bono work.'
Mr Pilgrim joined Kent CC as an accountancy trainee in 1968, and left in 1981 to become under secretary, finance, with the former Association of Metropolitan Authorities.
London Councils chair Merrick Cockell (Con) said: 'Martin Pilgrim's working life has been devoted to local government.
'He helped bring together the politically divided London borough bodies into a single organisation and made it work.'