Trenchard joins the agency after 16 years at Morgan Stanley, where he held a number of senior roles, including MD of UK and European corporate broking and global head of Asian equity sales.
His appointment as a senior adviser brings the agency's team to 18 and follows the hire of former Sunday Times City editor Kirstie Hamilton last April.
Grant said: 'It's been a priority for the last year to add to the senior consulting team here.
'Kirstie's appointment was the first stage of that and she brought us a City editor's view and David will bring a capital markets view,' he added.
Trenchard, who is set to join on 1 April, previously worked with Grant on mutual clients, including Scottish Power.
Trenchard said he found his work as a corporate broking adviser increasingly overlapping with City comms: 'A large part of the advisory work I'd been doing was on communications issues, helping companies construct their messages for the City and regulatory news service announcements.
'There were many areas where I found I was working alongside Andrew and the advisory work we were doing was of a similar nature.'
He added his experience 'on both sides of the Chinese Walls' - in both corporate broking and equity sales - would also bring value to the Tulchan offering.
Prior to working for Morgan Stanley in Asia, Trenchard acted as the group's European head of US equity sales and European head of global emerging markets equity sales.
Grant confirmed he aims to further build the agency's senior consultancy team over the year ahead.